diff --git a/tests/finance/test_blotter.py b/tests/finance/test_blotter.py
index 2a031c81..adfc5eee 100644
--- a/tests/finance/test_blotter.py
+++ b/tests/finance/test_blotter.py
@@ -154,7 +154,7 @@ class BlotterTestCase(WithCreateBarData,
         blotter = Blotter('minute', self.asset_finder,
                           cancel_policy=EODCancel())
 
-        # Make two orders for the same sid, so we can test that we are not
+        # Make two orders for the same asset, so we can test that we are not
         # mutating the orders list as we are cancelling orders
         blotter.order(self.asset_24, 100, MarketOrder())
         blotter.order(self.asset_24, -100, MarketOrder())
@@ -343,7 +343,7 @@ class BlotterTestCase(WithCreateBarData,
 
         other_order = Order(
             dt=blotter.current_dt,
-            sid=self.asset_25,
+            asset=self.asset_25,
             amount=1
         )
 
@@ -406,7 +406,7 @@ class BlotterTestCase(WithCreateBarData,
         equity_txn = txns[0]
         self.assertEqual(
             equity_txn.price,
-            bar_data.current(equity_txn.sid, 'price'),
+            bar_data.current(equity_txn.asset, 'price'),
         )
         self.assertEqual(commissions[0]['cost'], 1.0)
 
@@ -416,6 +416,6 @@ class BlotterTestCase(WithCreateBarData,
         future_txn = txns[1]
         self.assertEqual(
             future_txn.price,
-            bar_data.current(future_txn.sid, 'price') + 1.0,
+            bar_data.current(future_txn.asset, 'price') + 1.0,
         )
         self.assertEqual(commissions[1]['cost'], 2.0)
diff --git a/tests/finance/test_commissions.py b/tests/finance/test_commissions.py
index dacfe562..7bedb51a 100644
--- a/tests/finance/test_commissions.py
+++ b/tests/finance/test_commissions.py
@@ -21,16 +21,16 @@ class CommissionUnitTests(WithAssetFinder, ZiplineTestCase):
         asset1 = self.asset_finder.retrieve_asset(1)
 
         # one order
-        order = Order(dt=None, sid=asset1, amount=500)
+        order = Order(dt=None, asset=asset1, amount=500)
 
         # three fills
-        txn1 = Transaction(sid=asset1, amount=230, dt=None,
+        txn1 = Transaction(asset=asset1, amount=230, dt=None,
                            price=100, order_id=order.id)
 
-        txn2 = Transaction(sid=asset1, amount=170, dt=None,
+        txn2 = Transaction(asset=asset1, amount=170, dt=None,
                            price=101, order_id=order.id)
 
-        txn3 = Transaction(sid=asset1, amount=100, dt=None,
+        txn3 = Transaction(asset=asset1, amount=100, dt=None,
                            price=102, order_id=order.id)
 
         return order, [txn1, txn2, txn3]
diff --git a/tests/finance/test_slippage.py b/tests/finance/test_slippage.py
index 5698080f..85b07a73 100644
--- a/tests/finance/test_slippage.py
+++ b/tests/finance/test_slippage.py
@@ -126,7 +126,7 @@ class SlippageTestCase(WithCreateBarData,
                 'dt': datetime.datetime(2006, 1, 5, 14, 30, tzinfo=pytz.utc),
                 'amount': 100,
                 'filled': 0,
-                'sid': self.ASSET133,
+                'asset': self.ASSET133,
                 'limit': 3.5})
         ]
 
@@ -148,7 +148,7 @@ class SlippageTestCase(WithCreateBarData,
                 'dt': datetime.datetime(2006, 1, 5, 14, 30, tzinfo=pytz.utc),
                 'amount': 100,
                 'filled': 0,
-                'sid': self.ASSET133,
+                'asset': self.ASSET133,
                 'limit': 3.5})
         ]
 
@@ -170,7 +170,7 @@ class SlippageTestCase(WithCreateBarData,
                 'dt': datetime.datetime(2006, 1, 5, 14, 30, tzinfo=pytz.utc),
                 'amount': 100,
                 'filled': 0,
-                'sid': self.ASSET133,
+                'asset': self.ASSET133,
                 'limit': 3.6})
         ]
 
@@ -194,7 +194,7 @@ class SlippageTestCase(WithCreateBarData,
             # we ordered 100 shares, but default volume slippage only allows
             # for 2.5% of the volume.  2.5% * 2000 = 50 shares
             'amount': int(50),
-            'sid': int(133),
+            'asset': self.ASSET133,
             'order_id': open_orders[0].id
         }
 
@@ -209,7 +209,7 @@ class SlippageTestCase(WithCreateBarData,
                 'dt': datetime.datetime(2006, 1, 5, 14, 30, tzinfo=pytz.utc),
                 'amount': -100,
                 'filled': 0,
-                'sid': self.ASSET133,
+                'asset': self.ASSET133,
                 'limit': 3.5})
         ]
 
@@ -231,7 +231,7 @@ class SlippageTestCase(WithCreateBarData,
                 'dt': datetime.datetime(2006, 1, 5, 14, 30, tzinfo=pytz.utc),
                 'amount': -100,
                 'filled': 0,
-                'sid': self.ASSET133,
+                'asset': self.ASSET133,
                 'limit': 3.5})
         ]
 
@@ -253,7 +253,7 @@ class SlippageTestCase(WithCreateBarData,
                 'dt': datetime.datetime(2006, 1, 5, 14, 30, tzinfo=pytz.utc),
                 'amount': -100,
                 'filled': 0,
-                'sid': self.ASSET133,
+                'asset': self.ASSET133,
                 'limit': 3.4})
         ]
 
@@ -275,7 +275,7 @@ class SlippageTestCase(WithCreateBarData,
             'dt': datetime.datetime(
                 2006, 1, 5, 14, 32, tzinfo=pytz.utc),
             'amount': int(-50),
-            'sid': int(133)
+            'asset': self.ASSET133,
         }
 
         self.assertIsNotNone(txn)
@@ -293,7 +293,7 @@ class SlippageTestCase(WithCreateBarData,
                 'dt': datetime.datetime(2006, 1, 5, 14, 30, tzinfo=pytz.utc),
                 'amount': 100,
                 'filled': 0,
-                'sid': self.ASSET133,
+                'asset': self.ASSET133,
                 'stop': 4.0,
                 'limit': 3.0})
         ]
@@ -328,7 +328,7 @@ class SlippageTestCase(WithCreateBarData,
                 'dt': datetime.datetime(2006, 1, 5, 14, 30, tzinfo=pytz.utc),
                 'amount': 100,
                 'filled': 0,
-                'sid': self.ASSET133,
+                'asset': self.ASSET133,
                 'stop': 4.0,
                 'limit': 3.5})
         ]
@@ -363,7 +363,7 @@ class SlippageTestCase(WithCreateBarData,
                 'dt': datetime.datetime(2006, 1, 5, 14, 30, tzinfo=pytz.utc),
                 'amount': 100,
                 'filled': 0,
-                'sid': self.ASSET133,
+                'asset': self.ASSET133,
                 'stop': 4.0,
                 'limit': 3.6})
         ]
@@ -398,7 +398,7 @@ class SlippageTestCase(WithCreateBarData,
             'dt': datetime.datetime(
                 2006, 1, 5, 14, 34, tzinfo=pytz.utc),
             'amount': int(50),
-            'sid': int(133)
+            'asset': self.ASSET133
         }
 
         for key, value in expected_txn.items():
@@ -411,7 +411,7 @@ class SlippageTestCase(WithCreateBarData,
                 'dt': datetime.datetime(2006, 1, 5, 14, 30, tzinfo=pytz.utc),
                 'amount': -100,
                 'filled': 0,
-                'sid': self.ASSET133,
+                'asset': self.ASSET133,
                 'stop': 3.0,
                 'limit': 4.0})
         ]
@@ -446,7 +446,7 @@ class SlippageTestCase(WithCreateBarData,
                 'dt': datetime.datetime(2006, 1, 5, 14, 30, tzinfo=pytz.utc),
                 'amount': -100,
                 'filled': 0,
-                'sid': self.ASSET133,
+                'asset': self.ASSET133,
                 'stop': 3.0,
                 'limit': 3.5})
         ]
@@ -481,7 +481,7 @@ class SlippageTestCase(WithCreateBarData,
                 'dt': datetime.datetime(2006, 1, 5, 14, 30, tzinfo=pytz.utc),
                 'amount': -100,
                 'filled': 0,
-                'sid': self.ASSET133,
+                'asset': self.ASSET133,
                 'stop': 3.0,
                 'limit': 3.4})
         ]
@@ -516,7 +516,7 @@ class SlippageTestCase(WithCreateBarData,
             'dt': datetime.datetime(
                 2006, 1, 5, 14, 32, tzinfo=pytz.utc),
             'amount': int(-50),
-            'sid': int(133)
+            'asset': self.ASSET133,
         }
 
         for key, value in expected_txn.items():
@@ -574,7 +574,7 @@ class VolumeShareSlippageTestCase(WithCreateBarData,
                 dt=datetime.datetime(2006, 1, 5, 14, 30, tzinfo=pytz.utc),
                 amount=100,
                 filled=0,
-                sid=self.ASSET133
+                asset=self.ASSET133
             )
         ]
 
@@ -596,7 +596,7 @@ class VolumeShareSlippageTestCase(WithCreateBarData,
             'dt': datetime.datetime(
                 2006, 1, 5, 14, 31, tzinfo=pytz.utc),
             'amount': int(5),
-            'sid': int(133),
+            'asset': self.ASSET133,
             'commission': None,
             'type': DATASOURCE_TYPE.TRANSACTION,
             'order_id': open_orders[0].id
@@ -613,7 +613,7 @@ class VolumeShareSlippageTestCase(WithCreateBarData,
                 dt=datetime.datetime(2006, 1, 5, 14, 30, tzinfo=pytz.utc),
                 amount=100,
                 filled=0,
-                sid=self.ASSET133
+                asset=self.ASSET133
             )
         ]
 
@@ -684,7 +684,6 @@ class OrdersStopTestCase(WithSimParams,
                 'dt': pd.Timestamp('2006-01-05 14:30', tz='UTC'),
                 'amount': 100,
                 'filled': 0,
-                'sid': 133,
                 'stop': 3.5
             },
             'event': {
@@ -693,7 +692,6 @@ class OrdersStopTestCase(WithSimParams,
                 'price': 4.0,
                 'high': 3.15,
                 'low': 2.85,
-                'sid': 133,
                 'close': 4.0,
                 'open': 3.5
             },
@@ -702,7 +700,6 @@ class OrdersStopTestCase(WithSimParams,
                     'price': 4.00025,
                     'dt': pd.Timestamp('2006-01-05 14:31', tz='UTC'),
                     'amount': 50,
-                    'sid': 133,
                 }
             }
         },
@@ -711,7 +708,6 @@ class OrdersStopTestCase(WithSimParams,
                 'dt': pd.Timestamp('2006-01-05 14:30', tz='UTC'),
                 'amount': 100,
                 'filled': 0,
-                'sid': 133,
                 'stop': 3.6
             },
             'event': {
@@ -720,7 +716,6 @@ class OrdersStopTestCase(WithSimParams,
                 'price': 3.5,
                 'high': 3.15,
                 'low': 2.85,
-                'sid': 133,
                 'close': 3.5,
                 'open': 4.0
             },
@@ -733,7 +728,6 @@ class OrdersStopTestCase(WithSimParams,
                 'dt': pd.Timestamp('2006-01-05 14:30', tz='UTC'),
                 'amount': -100,
                 'filled': 0,
-                'sid': 133,
                 'stop': 3.4
             },
             'event': {
@@ -742,7 +736,6 @@ class OrdersStopTestCase(WithSimParams,
                 'price': 3.5,
                 'high': 3.15,
                 'low': 2.85,
-                'sid': 133,
                 'close': 3.5,
                 'open': 3.0
             },
@@ -755,7 +748,6 @@ class OrdersStopTestCase(WithSimParams,
                 'dt': pd.Timestamp('2006-01-05 14:30', tz='UTC'),
                 'amount': -100,
                 'filled': 0,
-                'sid': 133,
                 'stop': 3.5
             },
             'event': {
@@ -764,7 +756,6 @@ class OrdersStopTestCase(WithSimParams,
                 'price': 3.0,
                 'high': 3.15,
                 'low': 2.85,
-                'sid': 133,
                 'close': 3.0,
                 'open': 3.0
             },
@@ -773,7 +764,6 @@ class OrdersStopTestCase(WithSimParams,
                     'price': 2.9998125,
                     'dt': pd.Timestamp('2006-01-05 14:31', tz='UTC'),
                     'amount': -50,
-                    'sid': 133,
                 }
             }
         },
@@ -785,9 +775,13 @@ class OrdersStopTestCase(WithSimParams,
     ])
     def test_orders_stop(self, name, order_data, event_data, expected):
         data = order_data
-        data['sid'] = self.ASSET133
+        data['asset'] = self.ASSET133
         order = Order(**data)
 
+        if expected['transaction']:
+            expected['transaction']['asset'] = self.ASSET133
+        event_data['asset'] = self.ASSET133
+
         assets = (
             (133, pd.DataFrame(
                 {
diff --git a/tests/test_algorithm.py b/tests/test_algorithm.py
index 8a907fd2..cb98d12a 100644
--- a/tests/test_algorithm.py
+++ b/tests/test_algorithm.py
@@ -3802,7 +3802,7 @@ class TestOrderCancelation(WithDataPortal,
                     np.copysign(389, direction),
                     daily_positions[0]["amount"],
                 )
-                self.assertEqual(1, results.positions[0][0]["sid"].sid)
+                self.assertEqual(1, results.positions[0][0]["sid"])
 
             # should be an order on day1, but no more orders afterwards
             np.testing.assert_array_equal([1, 0, 0],
@@ -4067,7 +4067,6 @@ class TestEquityAutoClose(WithTmpDir, WithTradingCalendars, ZiplineTestCase):
         resources = self.make_data(auto_close_delta, 'daily', capital_base)
 
         assets = resources.assets
-        sids = [asset.sid for asset in assets]
         final_prices = resources.final_prices
 
         # Prices at which we expect our orders to be filled.
@@ -4174,14 +4173,14 @@ class TestEquityAutoClose(WithTmpDir, WithTradingCalendars, ZiplineTestCase):
                 self.test_days[1]
             )[1]
 
-        for sid, txn in zip(sids, initial_fills):
+        for asset, txn in zip(assets, initial_fills):
             self.assertDictContainsSubset(
                 {
                     'amount': order_size,
                     'commission': None,
                     'dt': last_minute_of_session,
-                    'price': initial_fill_prices[sid],
-                    'sid': sid,
+                    'price': initial_fill_prices[asset],
+                    'sid': asset,
                 },
                 txn,
             )
@@ -4203,7 +4202,7 @@ class TestEquityAutoClose(WithTmpDir, WithTradingCalendars, ZiplineTestCase):
                 'commission': 0.0,
                 'dt': assets[0].auto_close_date,
                 'price': fp0,
-                'sid': sids[0],
+                'sid': assets[0],
                 'order_id': None,  # Auto-close txns emit Nones for order_id.
             },
         )
@@ -4218,7 +4217,7 @@ class TestEquityAutoClose(WithTmpDir, WithTradingCalendars, ZiplineTestCase):
                 'commission': 0.0,
                 'dt': assets[1].auto_close_date,
                 'price': fp1,
-                'sid': sids[1],
+                'sid': assets[1],
                 'order_id': None,  # Auto-close txns emit Nones for order_id.
             },
         )
@@ -4313,7 +4312,6 @@ class TestEquityAutoClose(WithTmpDir, WithTradingCalendars, ZiplineTestCase):
 
         env = resources.env
         assets = resources.assets
-        sids = [a.sid for a in assets]
         final_prices = resources.final_prices
         backtest_minutes = resources.trade_data_by_sid[0].index.tolist()
 
@@ -4390,14 +4388,14 @@ class TestEquityAutoClose(WithTmpDir, WithTradingCalendars, ZiplineTestCase):
         # the backtest, which is still on the first day in minute mode.
         initial_fills = transactions.iloc[0]
         self.assertEqual(len(initial_fills), len(assets))
-        for sid, txn in zip(sids, initial_fills):
+        for asset, txn in zip(assets, initial_fills):
             self.assertDictContainsSubset(
                 {
                     'amount': order_size,
                     'commission': None,
                     'dt': backtest_minutes[1],
-                    'price': initial_fill_prices[sid],
-                    'sid': sid,
+                    'price': initial_fill_prices[asset],
+                    'sid': asset,
                 },
                 txn,
             )
@@ -4419,7 +4417,7 @@ class TestEquityAutoClose(WithTmpDir, WithTradingCalendars, ZiplineTestCase):
                 'commission': 0.0,
                 'dt': assets[0].auto_close_date,
                 'price': fp0,
-                'sid': sids[0],
+                'sid': assets[0],
                 'order_id': None,  # Auto-close txns emit Nones for order_id.
             },
         )
@@ -4434,7 +4432,7 @@ class TestEquityAutoClose(WithTmpDir, WithTradingCalendars, ZiplineTestCase):
                 'commission': 0.0,
                 'dt': assets[1].auto_close_date,
                 'price': fp1,
-                'sid': sids[1],
+                'sid': assets[1],
                 'order_id': None,  # Auto-close txns emit Nones for order_id.
             },
         )
diff --git a/tests/test_finance.py b/tests/test_finance.py
index 5b72449f..f176d608 100644
--- a/tests/test_finance.py
+++ b/tests/test_finance.py
@@ -187,8 +187,8 @@ class FinanceTestCase(WithLogger,
         # if present, expect transaction amounts to match orders exactly.
         complete_fill = params.get('complete_fill')
 
-        sid = 1
-        metadata = make_simple_equity_info([sid], self.start, self.end)
+        asset1 = self.asset_finder.retrieve_asset(1)
+        metadata = make_simple_equity_info([asset1.sid], self.start, self.end)
         with TempDirectory() as tempdir, \
                 tmp_trading_env(equities=metadata) as env:
 
@@ -206,7 +206,7 @@ class FinanceTestCase(WithLogger,
 
                 price_data = np.array([10.1] * len(minutes))
                 assets = {
-                    sid: pd.DataFrame({
+                    asset1.sid: pd.DataFrame({
                         "open": price_data,
                         "high": price_data,
                         "low": price_data,
@@ -305,7 +305,7 @@ class FinanceTestCase(WithLogger,
                     # place an order
                     direction = alternator ** len(order_list)
                     order_id = blotter.order(
-                        blotter.asset_finder.retrieve_asset(sid),
+                        asset1,
                         order_amount * direction,
                         MarketOrder())
                     order_list.append(blotter.orders[order_id])
@@ -333,7 +333,7 @@ class FinanceTestCase(WithLogger,
 
             for i in range(order_count):
                 order = order_list[i]
-                self.assertEqual(order.sid, sid)
+                self.assertEqual(order.asset, asset1)
                 self.assertEqual(order.amount, order_amount * alternator ** i)
 
             if complete_fill:
@@ -351,15 +351,19 @@ class FinanceTestCase(WithLogger,
 
             self.assertEqual(len(transactions), expected_txn_count)
 
-            cumulative_pos = tracker.position_tracker.positions[sid]
+            cumulative_pos = tracker.position_tracker.positions[asset1]
             if total_volume == 0:
                 self.assertIsNone(cumulative_pos)
             else:
                 self.assertEqual(total_volume, cumulative_pos.amount)
 
-            # the open orders should not contain sid.
+            # the open orders should not contain the asset.
             oo = blotter.open_orders
-            self.assertNotIn(sid, oo, "Entry is removed when no open orders")
+            self.assertNotIn(
+                asset1,
+                oo,
+                "Entry is removed when no open orders"
+            )
 
     def test_blotter_processes_splits(self):
         blotter = Blotter('daily', self.env.asset_finder,
@@ -376,23 +380,24 @@ class FinanceTestCase(WithLogger,
         blotter.process_splits([(2, 0.3333)])
 
         for sid in [1, 2]:
-            order_lists = blotter.open_orders[sid]
+            order_lists = \
+                blotter.open_orders[blotter.asset_finder.retrieve_asset(sid)]
             self.assertIsNotNone(order_lists)
             self.assertEqual(1, len(order_lists))
 
-        aapl_order = blotter.open_orders[1][0].to_dict()
-        fls_order = blotter.open_orders[2][0].to_dict()
+        aapl_order = blotter.open_orders[1][0]
+        fls_order = blotter.open_orders[2][0]
 
         # make sure the aapl order didn't change
-        self.assertEqual(100, aapl_order['amount'])
-        self.assertEqual(10, aapl_order['limit'])
-        self.assertEqual(1, aapl_order['sid'])
+        self.assertEqual(100, aapl_order.amount)
+        self.assertEqual(10, aapl_order.limit)
+        self.assertEqual(1, aapl_order.asset)
 
         # make sure the fls order did change
         # to 300 shares at 3.33
-        self.assertEqual(300, fls_order['amount'])
-        self.assertEqual(3.33, fls_order['limit'])
-        self.assertEqual(2, fls_order['sid'])
+        self.assertEqual(300, fls_order.amount)
+        self.assertEqual(3.33, fls_order.limit)
+        self.assertEqual(2, fls_order.asset)
 
 
 class TradingEnvironmentTestCase(WithLogger,
diff --git a/tests/test_perf_tracking.py b/tests/test_perf_tracking.py
index f76f9ee5..f0ac8af6 100644
--- a/tests/test_perf_tracking.py
+++ b/tests/test_perf_tracking.py
@@ -347,7 +347,7 @@ class TestSplitPerformance(WithSimParams, WithTmpDir, ZiplineTestCase):
         # check the last position to make sure it's been updated
         position = latest_positions[0]
 
-        self.assertEqual(1, position['sid'])
+        self.assertEqual(self.asset1, position['sid'])
         self.assertEqual(33, position['amount'])
         self.assertEqual(60, position['cost_basis'])
         self.assertEqual(60, position['last_sale_price'])
@@ -1326,8 +1326,8 @@ class TestPositionPerformance(WithInstanceTmpDir, WithTradingCalendars,
             "should be just one position")
 
         self.assertEqual(
-            pp.positions[1].sid,
-            txn.sid,
+            pp.positions[1].asset,
+            txn.asset,
             "position should be in security with id 1")
 
         self.assertEqual(
@@ -1437,8 +1437,8 @@ single short-sale transaction"""
             "should be just one position")
 
         self.assertEqual(
-            pp.positions[1].sid,
-            txn.sid,
+            pp.positions[1].asset,
+            txn.asset,
             "position should be in security from the transaction"
         )
 
@@ -1494,8 +1494,8 @@ single short-sale transaction"""
         )
 
         self.assertEqual(
-            pp.positions[1].sid,
-            txn.sid,
+            pp.positions[1].asset,
+            txn.asset,
             "position should be in security from the transaction"
         )
 
@@ -1556,8 +1556,8 @@ cost of sole txn in test"
             "should be just one position"
         )
         self.assertEqual(
-            ppTotal.positions[1].sid,
-            txn.sid,
+            ppTotal.positions[1].asset,
+            txn.asset,
             "position should be in security from the transaction"
         )
 
@@ -1834,7 +1834,7 @@ shares in position"
         self.create_environment_stuff(num_days=8)
 
         history_args = (
-            1,
+            self.asset1,
             [10, 9, 11, 8, 9, 12, 13, 14],
             [200, -100, -100, 100, -300, 100, 500, 400],
             oneday,
@@ -1973,6 +1973,16 @@ class TestPositionTracker(WithTradingEnvironment,
                           ZiplineTestCase):
     ASSET_FINDER_EQUITY_SIDS = 1, 2
 
+    @classmethod
+    def init_class_fixtures(cls):
+        super(TestPositionTracker, cls).init_class_fixtures()
+
+        cls.EQUITY1 = cls.asset_finder.retrieve_asset(1)
+        cls.EQUITY2 = cls.asset_finder.retrieve_asset(2)
+        cls.FUTURE3 = cls.asset_finder.retrieve_asset(3)
+        cls.FUTURE4 = cls.asset_finder.retrieve_asset(4)
+        cls.FUTURE5 = cls.asset_finder.retrieve_asset(1032201401)
+
     @classmethod
     def make_futures_info(cls):
         return pd.DataFrame.from_dict(
@@ -2017,13 +2027,13 @@ class TestPositionTracker(WithTradingEnvironment,
     def test_position_values_and_exposures(self):
         pt = perf.PositionTracker(self.env.asset_finder, None)
         dt = pd.Timestamp("1984/03/06 3:00PM")
-        pos1 = perf.Position(1, amount=np.float64(10.0),
+        pos1 = perf.Position(self.EQUITY1, amount=np.float64(10.0),
                              last_sale_date=dt, last_sale_price=10)
-        pos2 = perf.Position(2, amount=np.float64(-20.0),
+        pos2 = perf.Position(self.EQUITY2, amount=np.float64(-20.0),
                              last_sale_date=dt, last_sale_price=10)
-        pos3 = perf.Position(3, amount=np.float64(30.0),
+        pos3 = perf.Position(self.FUTURE3, amount=np.float64(30.0),
                              last_sale_date=dt, last_sale_price=10)
-        pos4 = perf.Position(4, amount=np.float64(-40.0),
+        pos4 = perf.Position(self.FUTURE4, amount=np.float64(-40.0),
                              last_sale_date=dt, last_sale_price=10)
         pt.update_positions({1: pos1, 2: pos2, 3: pos3, 4: pos4})
 
@@ -2049,11 +2059,11 @@ class TestPositionTracker(WithTradingEnvironment,
     def test_update_positions(self):
         pt = perf.PositionTracker(self.env.asset_finder, None)
         dt = pd.Timestamp("2014/01/01 3:00PM")
-        pos1 = perf.Position(1, amount=np.float64(10.0),
+        pos1 = perf.Position(self.EQUITY1, amount=np.float64(10.0),
                              last_sale_date=dt, last_sale_price=10)
-        pos2 = perf.Position(2, amount=np.float64(-20.0),
+        pos2 = perf.Position(self.EQUITY2, amount=np.float64(-20.0),
                              last_sale_date=dt, last_sale_price=10)
-        pos3 = perf.Position(1032201401, amount=np.float64(30.0),
+        pos3 = perf.Position(self.FUTURE5, amount=np.float64(30.0),
                              last_sale_date=dt, last_sale_price=100)
 
         # Call update_positions twice. When the second call is made,
@@ -2063,8 +2073,8 @@ class TestPositionTracker(WithTradingEnvironment,
         # were to be stored as a dict, then its order could change in arbitrary
         # ways when the second update_positions call is made. Hence we also
         # store it as an OrderedDict.
-        pt.update_positions({1: pos1, 1032201401: pos3})
-        pt.update_positions({2: pos2})
+        pt.update_positions({self.EQUITY1: pos1, self.FUTURE5: pos3})
+        pt.update_positions({self.EQUITY2: pos2})
 
         pos_stats = pt.stats()
         # Test long-only methods
@@ -2087,19 +2097,17 @@ class TestPositionTracker(WithTradingEnvironment,
         self.assertEqual(100 + 150000 - 200, pos_stats.net_exposure)
 
     def test_close_position(self):
-        future_sid = 1032201401
-        equity_sid = 1
         pt = perf.PositionTracker(self.env.asset_finder, None)
         dt = pd.Timestamp('2017/01/04 3:00PM')
 
         pos1 = perf.Position(
-            sid=future_sid,
+            asset=self.FUTURE5,
             amount=np.float64(30.0),
             last_sale_date=dt,
             last_sale_price=100,
         )
         pos2 = perf.Position(
-            sid=equity_sid,
+            asset=self.EQUITY1,
             amount=np.float64(10.0),
             last_sale_date=dt,
             last_sale_price=10,
@@ -2110,10 +2118,9 @@ class TestPositionTracker(WithTradingEnvironment,
         # OrderedDicts. If `future_sid` is not removed from the multipliers
         # dictionaries, equities will hit the incorrect multiplier when
         # computing `pt.stats()`.
-        pt.update_positions({future_sid: pos1, equity_sid: pos2})
+        pt.update_positions({self.FUTURE5: pos1, self.EQUITY1: pos2})
 
-        asset_to_close = self.env.asset_finder.retrieve_asset(future_sid)
-        txn = create_txn(asset_to_close, dt, 100, -30)
+        txn = create_txn(self.FUTURE5, dt, 100, -30)
         pt.execute_transaction(txn)
 
         pos_stats = pt.stats()
diff --git a/zipline/finance/blotter.py b/zipline/finance/blotter.py
index 9a61e06a..9ca934b6 100644
--- a/zipline/finance/blotter.py
+++ b/zipline/finance/blotter.py
@@ -18,7 +18,7 @@ from copy import copy
 
 from six import iteritems
 
-from zipline.assets import Equity, Future
+from zipline.assets import Equity, Future, Asset
 from zipline.finance.order import Order
 from zipline.finance.slippage import VolumeShareSlippage
 from zipline.finance.commission import (
@@ -27,6 +27,7 @@ from zipline.finance.commission import (
     PerTrade,
 )
 from zipline.finance.cancel_policy import NeverCancel
+from zipline.utils.input_validation import expect_types
 
 log = Logger('Blotter')
 warning_logger = Logger('AlgoWarning')
@@ -36,15 +37,12 @@ class Blotter(object):
     def __init__(self, data_frequency, asset_finder, equity_slippage=None,
                  future_slippage=None, equity_commission=None,
                  future_commission=None, cancel_policy=None):
-        # these orders are aggregated by sid
+        # these orders are aggregated by asset
         self.open_orders = defaultdict(list)
 
         # keep a dict of orders by their own id
         self.orders = {}
 
-        # all our legacy order management code works with integer sids.
-        # this lets us convert those to assets when needed.  ideally, we'd just
-        # revamp all the legacy code to work with assets.
         self.asset_finder = asset_finder
 
         # holding orders that have come in since the last event.
@@ -88,7 +86,8 @@ class Blotter(object):
     def set_date(self, dt):
         self.current_dt = dt
 
-    def order(self, sid, amount, style, order_id=None):
+    @expect_types(asset=Asset)
+    def order(self, asset, amount, style, order_id=None):
         """Place an order.
 
         Parameters
@@ -114,10 +113,10 @@ class Blotter(object):
         -----
         amount > 0 :: Buy/Cover
         amount < 0 :: Sell/Short
-        Market order:    order(sid, amount)
-        Limit order:     order(sid, amount, style=LimitOrder(limit_price))
-        Stop order:      order(sid, amount, style=StopOrder(stop_price))
-        StopLimit order: order(sid, amount, style=StopLimitOrder(limit_price,
+        Market order:    order(asset, amount)
+        Limit order:     order(asset, amount, style=LimitOrder(limit_price))
+        Stop order:      order(asset, amount, style=StopOrder(stop_price))
+        StopLimit order: order(asset, amount, style=StopLimitOrder(limit_price,
                                stop_price))
         """
         # something could be done with amount to further divide
@@ -136,14 +135,14 @@ class Blotter(object):
         is_buy = (amount > 0)
         order = Order(
             dt=self.current_dt,
-            sid=sid,
+            asset=asset,
             amount=amount,
             stop=style.get_stop_price(is_buy),
             limit=style.get_limit_price(is_buy),
             id=order_id
         )
 
-        self.open_orders[order.sid].append(order)
+        self.open_orders[order.asset].append(order)
         self.orders[order.id] = order
         self.new_orders.append(order)
 
@@ -177,7 +176,7 @@ class Blotter(object):
         cur_order = self.orders[order_id]
 
         if cur_order.open:
-            order_list = self.open_orders[cur_order.sid]
+            order_list = self.open_orders[cur_order.asset]
             if cur_order in order_list:
                 order_list.remove(cur_order)
 
@@ -217,7 +216,7 @@ class Blotter(object):
                         'filled by the end of day and '
                         'were canceled.'.format(
                             order_amt=order.amount,
-                            order_sym=order.sid.symbol,
+                            order_sym=order.asset.symbol,
                             order_filled=order.filled,
                             order_failed=order.amount - order.filled,
                         )
@@ -231,7 +230,7 @@ class Blotter(object):
                         'filled by the end of day and '
                         'were canceled.'.format(
                             order_amt=order.amount,
-                            order_sym=order.sid.symbol,
+                            order_sym=order.asset.symbol,
                             order_filled=-1 * order.filled,
                             order_failed=-1 * (order.amount - order.filled),
                         )
@@ -242,7 +241,7 @@ class Blotter(object):
                         '{order_sym} failed to fill by the end of day '
                         'and was canceled.'.format(
                             order_amt=order.amount,
-                            order_sym=order.sid.symbol,
+                            order_sym=order.asset.symbol,
                         )
                     )
 
@@ -268,7 +267,7 @@ class Blotter(object):
 
         cur_order = self.orders[order_id]
 
-        order_list = self.open_orders[cur_order.sid]
+        order_list = self.open_orders[cur_order.asset]
         if cur_order in order_list:
             order_list.remove(cur_order)
 
@@ -306,20 +305,19 @@ class Blotter(object):
         Parameters
         ----------
         splits: list
-            A list of splits.  Each split is a tuple of (sid, ratio).
+            A list of splits.  Each split is a tuple of (asset, ratio).
 
         Returns
         -------
         None
         """
-        for split in splits:
-            sid = split[0]
-            if sid not in self.open_orders:
+        for asset, ratio in splits:
+            if asset not in self.open_orders:
                 return
 
-            orders_to_modify = self.open_orders[sid]
+            orders_to_modify = self.open_orders[asset]
             for order in orders_to_modify:
-                order.handle_split(split[1])
+                order.handle_split(ratio)
 
     def get_transactions(self, bar_data):
         """
@@ -344,7 +342,7 @@ class Blotter(object):
 
         commissions_list: List
             commissions_list: list of commissions resulting from filling the
-            open orders.  A commission is an object with "sid" and "cost"
+            open orders.  A commission is an object with "asset" and "cost"
             parameters.
 
         closed_orders: List
@@ -356,11 +354,7 @@ class Blotter(object):
         commissions = []
 
         if self.open_orders:
-            assets = self.asset_finder.retrieve_all(self.open_orders)
-            asset_dict = {asset.sid: asset for asset in assets}
-
-            for sid, asset_orders in iteritems(self.open_orders):
-                asset = asset_dict[sid]
+            for asset, asset_orders in iteritems(self.open_orders):
                 slippage = self.slippage_models[type(asset)]
 
                 for order, txn in \
@@ -370,7 +364,7 @@ class Blotter(object):
 
                     if additional_commission > 0:
                         commissions.append({
-                            "sid": order.sid,
+                            "asset": order.asset,
                             "order": order,
                             "cost": additional_commission
                         })
@@ -401,15 +395,15 @@ class Blotter(object):
         """
         # remove all closed orders from our open_orders dict
         for order in closed_orders:
-            sid = order.sid
-            sid_orders = self.open_orders[sid]
+            asset = order.asset
+            asset_orders = self.open_orders[asset]
             try:
-                sid_orders.remove(order)
+                asset_orders.remove(order)
             except ValueError:
                 continue
 
-        # now clear out the sids from our open_orders dict that have
+        # now clear out the assets from our open_orders dict that have
         # zero open orders
-        for sid in list(self.open_orders.keys()):
-            if len(self.open_orders[sid]) == 0:
-                del self.open_orders[sid]
+        for asset in list(self.open_orders.keys()):
+            if len(self.open_orders[asset]) == 0:
+                del self.open_orders[asset]
diff --git a/zipline/finance/order.py b/zipline/finance/order.py
index d326f206..91b3edae 100644
--- a/zipline/finance/order.py
+++ b/zipline/finance/order.py
@@ -20,6 +20,7 @@ from six import text_type
 import zipline.protocol as zp
 from zipline.assets import Asset
 from zipline.utils.enum import enum
+from zipline.utils.input_validation import expect_types
 
 ORDER_STATUS = enum(
     'OPEN',
@@ -34,35 +35,35 @@ BUY = 1 << 1
 STOP = 1 << 2
 LIMIT = 1 << 3
 
-ORDER_FIELDS_TO_IGNORE = {'type', 'direction', '_status'}
+ORDER_FIELDS_TO_IGNORE = {'type', 'direction', '_status', 'asset'}
 
 
 class Order(object):
     # using __slots__ to save on memory usage.  Simulations can create many
     # Order objects and we keep them all in memory, so it's worthwhile trying
     # to cut down on the memory footprint of this object.
-    __slots__ = ["id", "dt", "reason", "created", "sid", "amount", "filled",
+    __slots__ = ["id", "dt", "reason", "created", "asset", "amount", "filled",
                  "commission", "_status", "stop", "limit", "stop_reached",
                  "limit_reached", "direction", "type", "broker_order_id"]
 
-    def __init__(self, dt, sid, amount, stop=None, limit=None, filled=0,
+    @expect_types(asset=Asset)
+    def __init__(self, dt, asset, amount, stop=None, limit=None, filled=0,
                  commission=0, id=None):
         """
         @dt - datetime.datetime that the order was placed
-        @sid - asset for the order.  called sid for historical reasons.
+        @asset - asset for the order.
         @amount - the number of shares to buy/sell
                   a positive sign indicates a buy
                   a negative sign indicates a sell
         @filled - how many shares of the order have been filled so far
         """
-        assert isinstance(sid, Asset)
 
         # get a string representation of the uuid.
         self.id = self.make_id() if id is None else id
         self.dt = dt
         self.reason = None
         self.created = dt
-        self.sid = sid
+        self.asset = asset
         self.amount = amount
         self.filled = filled
         self.commission = commission
@@ -86,9 +87,16 @@ class Order(object):
         if self.broker_order_id is None:
             del dct['broker_order_id']
 
+        dct['sid'] = self.asset
         dct['status'] = self.status
         return dct
 
+    @property
+    def sid(self):
+        # For backwards compatibility because we pass this object to
+        # custom slippage models.
+        return self.asset
+
     def to_api_obj(self):
         pydict = self.to_dict()
         obj = zp.Order(initial_values=pydict)
@@ -215,14 +223,6 @@ class Order(object):
     def open(self):
         return self.status in [ORDER_STATUS.OPEN, ORDER_STATUS.HELD]
 
-    @property
-    def asset(self):
-        """
-        Convenience accessor to hide away a historical API that we'd like to
-        change at some point.
-        """
-        return self.sid
-
     @property
     def triggered(self):
         """
diff --git a/zipline/finance/performance/period.py b/zipline/finance/performance/period.py
index cc17f374..1a0d8d70 100644
--- a/zipline/finance/performance/period.py
+++ b/zipline/finance/performance/period.py
@@ -185,8 +185,8 @@ class PerformancePeriod(object):
         self._account_store = zp.Account()
         self.serialize_positions = serialize_positions
 
-        # This dict contains the known cash flow multipliers for sids and is
-        # keyed on sid
+        # This dict contains the known cash flow multipliers for assets and is
+        # keyed on asset
         self._execution_cash_flow_multipliers = {}
 
     _position_tracker = None
@@ -362,7 +362,7 @@ class PerformancePeriod(object):
     def handle_execution(self, txn):
         self.cash_flow += self._calculate_execution_cash_flow(txn)
 
-        asset = self.asset_finder.retrieve_asset(txn.sid)
+        asset = self.asset_finder.retrieve_asset(txn.asset)
         if isinstance(asset, Future):
             try:
                 old_price = self._payout_last_sale_prices[asset]
@@ -392,15 +392,15 @@ class PerformancePeriod(object):
         # Check if the multiplier is cached. If it is not, look up the asset
         # and cache the multiplier.
         try:
-            multiplier = self._execution_cash_flow_multipliers[txn.sid]
+            multiplier = self._execution_cash_flow_multipliers[txn.asset]
         except KeyError:
-            asset = self.asset_finder.retrieve_asset(txn.sid)
+            asset = self.asset_finder.retrieve_asset(txn.asset)
             # Futures experience no cash flow on transactions
             if isinstance(asset, Future):
                 multiplier = 0
             else:
                 multiplier = 1
-            self._execution_cash_flow_multipliers[txn.sid] = multiplier
+            self._execution_cash_flow_multipliers[txn.asset] = multiplier
 
         # Calculate and return the cash flow given the multiplier
         return -1 * txn.price * txn.amount * multiplier
diff --git a/zipline/finance/performance/position.py b/zipline/finance/performance/position.py
index 7c818d59..24bc5bb4 100644
--- a/zipline/finance/performance/position.py
+++ b/zipline/finance/performance/position.py
@@ -20,7 +20,7 @@ Position Tracking
     +-----------------+----------------------------------------------------+
     | key             | value                                              |
     +=================+====================================================+
-    | sid             | the sid for the asset held in this position        |
+    | asset           | the asset held in this position                    |
     +-----------------+----------------------------------------------------+
     | amount          | whole number of shares in the position             |
     +-----------------+----------------------------------------------------+
@@ -37,15 +37,19 @@ from collections import OrderedDict
 import numpy as np
 import logbook
 
+from zipline.assets import Future, Asset
+from zipline.utils.input_validation import expect_types
+
 log = logbook.Logger('Performance')
 
 
 class Position(object):
 
-    def __init__(self, sid, amount=0, cost_basis=0.0,
+    @expect_types(asset=Asset)
+    def __init__(self, asset, amount=0, cost_basis=0.0,
                  last_sale_price=0.0, last_sale_date=None):
 
-        self.sid = sid
+        self.asset = asset
         self.amount = amount
         self.cost_basis = cost_basis  # per share
         self.last_sale_price = last_sale_price
@@ -72,15 +76,16 @@ class Position(object):
             )
         }
 
-    def handle_split(self, sid, ratio):
+    @expect_types(asset=Asset)
+    def handle_split(self, asset, ratio):
         """
         Update the position by the split ratio, and return the resulting
         fractional share that will be converted into cash.
 
         Returns the unused cash.
         """
-        if self.sid != sid:
-            raise Exception("updating split with the wrong sid!")
+        if self.asset != asset:
+            raise Exception("updating split with the wrong asset!")
 
         # adjust the # of shares by the ratio
         # (if we had 100 shares, and the ratio is 3,
@@ -113,9 +118,9 @@ class Position(object):
         return return_cash
 
     def update(self, txn):
-        if self.sid != txn.sid:
+        if self.asset != txn.asset:
             raise Exception('updating position with txn for a '
-                            'different sid')
+                            'different asset')
 
         total_shares = self.amount + txn.amount
 
@@ -145,7 +150,8 @@ class Position(object):
 
         self.amount = total_shares
 
-    def adjust_commission_cost_basis(self, sid, cost):
+    @expect_types(asset=Asset)
+    def adjust_commission_cost_basis(self, asset, cost):
         """
         A note about cost-basis in zipline: all positions are considered
         to share a cost basis, even if they were executed in different
@@ -156,8 +162,8 @@ class Position(object):
         all shares in a position.
         """
 
-        if sid != self.sid:
-            raise Exception('Updating a commission for a different sid?')
+        if asset != self.asset:
+            raise Exception('Updating a commission for a different asset?')
         if cost == 0.0:
             return
 
@@ -171,10 +177,10 @@ class Position(object):
         self.cost_basis = new_cost / self.amount
 
     def __repr__(self):
-        template = "sid: {sid}, amount: {amount}, cost_basis: {cost_basis}, \
+        template = "asset: {asset}, amount: {amount}, cost_basis: {cost_basis}, \
 last_sale_price: {last_sale_price}"
         return template.format(
-            sid=self.sid,
+            asset=self.asset,
             amount=self.amount,
             cost_basis=self.cost_basis,
             last_sale_price=self.last_sale_price
@@ -186,7 +192,7 @@ last_sale_price: {last_sale_price}"
         Returns a dict object of the form:
         """
         return {
-            'sid': self.sid,
+            'sid': self.asset,
             'amount': self.amount,
             'cost_basis': self.cost_basis,
             'last_sale_price': self.last_sale_price
diff --git a/zipline/finance/performance/position_tracker.py b/zipline/finance/performance/position_tracker.py
index 417c7392..0f8edc25 100644
--- a/zipline/finance/performance/position_tracker.py
+++ b/zipline/finance/performance/position_tracker.py
@@ -19,8 +19,6 @@ import logbook
 import numpy as np
 from collections import namedtuple
 from math import isnan
-from zipline.finance.performance.position import Position
-from zipline.finance.transaction import Transaction
 
 try:
     # optional cython based OrderedDict
@@ -29,9 +27,14 @@ except ImportError:
     from collections import OrderedDict
 from six import iteritems, itervalues
 
+from zipline.finance.performance.position import Position
+from zipline.finance.transaction import Transaction
+from zipline.utils.input_validation import expect_types
 import zipline.protocol as zp
 from zipline.assets import (
-    Equity, Future
+    Equity,
+    Future,
+    Asset
 )
 from zipline.errors import PositionTrackerMissingAssetFinder
 from . position import positiondict
@@ -122,7 +125,7 @@ class PositionTracker(object):
     def __init__(self, asset_finder, data_frequency):
         self.asset_finder = asset_finder
 
-        # sid => position object
+        # asset => position object
         self.positions = positiondict()
         # Arrays for quick calculations of positions value
         self._position_value_multipliers = OrderedDict()
@@ -133,41 +136,42 @@ class PositionTracker(object):
 
         self.data_frequency = data_frequency
 
-    def _update_asset(self, sid):
+    def _update_asset(self, asset):
         try:
-            self._position_value_multipliers[sid]
-            self._position_exposure_multipliers[sid]
+            self._position_value_multipliers[asset]
+            self._position_exposure_multipliers[asset]
         except KeyError:
             # Check if there is an AssetFinder
             if self.asset_finder is None:
                 raise PositionTrackerMissingAssetFinder()
 
-            # Collect the value multipliers from applicable sids
-            asset = self.asset_finder.retrieve_asset(sid)
+            # Collect the value multipliers from applicable assets
+            asset = self.asset_finder.retrieve_asset(asset)
             if isinstance(asset, Equity):
-                self._position_value_multipliers[sid] = 1
-                self._position_exposure_multipliers[sid] = 1
+                self._position_value_multipliers[asset] = 1
+                self._position_exposure_multipliers[asset] = 1
             if isinstance(asset, Future):
-                self._position_value_multipliers[sid] = 0
-                self._position_exposure_multipliers[sid] = asset.multiplier
+                self._position_value_multipliers[asset] = 0
+                self._position_exposure_multipliers[asset] = asset.multiplier
 
     def update_positions(self, positions):
         # update positions in batch
         self.positions.update(positions)
-        for sid, pos in iteritems(positions):
-            self._update_asset(sid)
+        for asset, pos in iteritems(positions):
+            self._update_asset(asset)
 
-    def update_position(self, sid, amount=None, last_sale_price=None,
+    @expect_types(asset=Asset)
+    def update_position(self, asset, amount=None, last_sale_price=None,
                         last_sale_date=None, cost_basis=None):
-        if sid not in self.positions:
-            position = Position(sid)
-            self.positions[sid] = position
+        if asset not in self.positions:
+            position = Position(asset)
+            self.positions[asset] = position
         else:
-            position = self.positions[sid]
+            position = self.positions[asset]
 
         if amount is not None:
             position.amount = amount
-            self._update_asset(sid=sid)
+            self._update_asset(asset=asset)
         if last_sale_price is not None:
             position.last_sale_price = last_sale_price
         if last_sale_date is not None:
@@ -178,36 +182,37 @@ class PositionTracker(object):
     def execute_transaction(self, txn):
         # Update Position
         # ----------------
-        sid = txn.sid
+        asset = txn.asset
 
-        if sid not in self.positions:
-            position = Position(sid)
-            self.positions[sid] = position
+        if asset not in self.positions:
+            position = Position(asset)
+            self.positions[asset] = position
         else:
-            position = self.positions[sid]
+            position = self.positions[asset]
 
         position.update(txn)
 
         if position.amount == 0:
             # if this position now has 0 shares, remove it from our internal
             # bookkeeping.
-            del self.positions[sid]
-            del self._position_value_multipliers[sid]
-            del self._position_exposure_multipliers[sid]
+            del self.positions[asset]
+            del self._position_value_multipliers[asset]
+            del self._position_exposure_multipliers[asset]
 
             try:
                 # if this position exists in our user-facing dictionary,
                 # remove it as well.
-                del self._positions_store[sid]
+                del self._positions_store[asset]
             except KeyError:
                 pass
         else:
-            self._update_asset(sid)
+            self._update_asset(asset)
 
-    def handle_commission(self, sid, cost):
+    @expect_types(asset=Asset)
+    def handle_commission(self, asset, cost):
         # Adjust the cost basis of the stock if we own it
-        if sid in self.positions:
-            self.positions[sid].adjust_commission_cost_basis(sid, cost)
+        if asset in self.positions:
+            self.positions[asset].adjust_commission_cost_basis(asset, cost)
 
     def handle_splits(self, splits):
         """
@@ -216,7 +221,8 @@ class PositionTracker(object):
         Parameters
         ----------
         splits: list
-            A list of splits.  Each split is a tuple of (sid, ratio).
+            A list of splits.  Each split is a tuple of (sid, ratio), where
+            sid is an integer.
 
         Returns
         -------
@@ -227,11 +233,12 @@ class PositionTracker(object):
 
         for split in splits:
             sid = split[0]
-            if sid in self.positions:
+            asset = self.asset_finder.retrieve_asset(sid)
+            if asset in self.positions:
                 # Make the position object handle the split. It returns the
                 # leftover cash from a fractional share, if there is any.
-                position = self.positions[sid]
-                leftover_cash = position.handle_split(sid, split[1])
+                position = self.positions[asset]
+                leftover_cash = position.handle_split(asset, split[1])
                 self._update_asset(split[0])
                 total_leftover_cash += leftover_cash
 
@@ -326,7 +333,7 @@ class PositionTracker(object):
             price = self.positions.get(asset).last_sale_price
 
         txn = Transaction(
-            sid=asset,
+            asset=asset,
             amount=(-1 * amount),
             dt=dt,
             price=price,
@@ -339,20 +346,20 @@ class PositionTracker(object):
 
         positions = self._positions_store
 
-        for sid, pos in iteritems(self.positions):
+        for asset, pos in iteritems(self.positions):
 
             if pos.amount == 0:
                 # Clear out the position if it has become empty since the last
                 # time get_positions was called.  Catching the KeyError is
-                # faster than checking `if sid in positions`, and this can be
+                # faster than checking `if asset in positions`, and this can be
                 # potentially called in a tight inner loop.
                 try:
-                    del positions[sid]
+                    del positions[asset]
                 except KeyError:
                     pass
                 continue
 
-            position = zp.Position(sid)
+            position = zp.Position(asset)
             position.amount = pos.amount
             position.cost_basis = pos.cost_basis
             position.last_sale_price = pos.last_sale_price
@@ -360,13 +367,13 @@ class PositionTracker(object):
 
             # Adds the new position if we didn't have one before, or overwrite
             # one we have currently
-            positions[sid] = position
+            positions[asset] = position
 
         return positions
 
     def get_positions_list(self):
         positions = []
-        for sid, pos in iteritems(self.positions):
+        for asset, pos in iteritems(self.positions):
             if pos.amount != 0:
                 positions.append(pos.to_dict())
         return positions
diff --git a/zipline/finance/performance/tracker.py b/zipline/finance/performance/tracker.py
index ffef7ecd..c7db49ee 100644
--- a/zipline/finance/performance/tracker.py
+++ b/zipline/finance/performance/tracker.py
@@ -275,10 +275,10 @@ class PerformanceTracker(object):
         self.todays_performance.record_order(event)
 
     def process_commission(self, commission):
-        sid = commission['sid']
+        asset = commission['asset']
         cost = commission['cost']
 
-        self.position_tracker.handle_commission(sid, cost)
+        self.position_tracker.handle_commission(asset, cost)
         self.cumulative_performance.handle_commission(cost)
         self.todays_performance.handle_commission(cost)
 
diff --git a/zipline/finance/transaction.py b/zipline/finance/transaction.py
index dcd2fce2..55814238 100644
--- a/zipline/finance/transaction.py
+++ b/zipline/finance/transaction.py
@@ -18,14 +18,13 @@ from copy import copy
 
 from zipline.assets import Asset
 from zipline.protocol import DATASOURCE_TYPE
+from zipline.utils.input_validation import expect_types
 
 
 class Transaction(object):
-
-    def __init__(self, sid, amount, dt, price, order_id, commission=None):
-        assert isinstance(sid, Asset)
-
-        self.sid = sid
+    @expect_types(asset=Asset)
+    def __init__(self, asset, amount, dt, price, order_id, commission=None):
+        self.asset = asset
         self.amount = amount
         self.dt = dt
         self.price = price
@@ -39,6 +38,9 @@ class Transaction(object):
     def to_dict(self):
         py = copy(self.__dict__)
         del py['type']
+        del py['asset']
+        py['sid'] = self.asset
+
         return py
 
 
@@ -53,7 +55,7 @@ def create_transaction(order, dt, price, amount):
         raise Exception("Transaction magnitude must be at least 1.")
 
     transaction = Transaction(
-        sid=order.sid,
+        asset=order.asset,
         amount=int(amount),
         dt=dt,
         price=price,
diff --git a/zipline/protocol.py b/zipline/protocol.py
index 8013251e..fd78a710 100644
--- a/zipline/protocol.py
+++ b/zipline/protocol.py
@@ -16,6 +16,8 @@ from warnings import warn
 
 import pandas as pd
 
+from zipline.assets import Asset
+from zipline.utils.input_validation import expect_types
 from .utils.enum import enum
 from zipline._protocol import BarData  # noqa
 
@@ -225,14 +227,17 @@ class Account(object):
 
 
 class Position(object):
-
-    def __init__(self, sid):
-        self.sid = sid
+    @expect_types(asset=Asset)
+    def __init__(self, asset):
+        self.asset = asset
         self.amount = 0
         self.cost_basis = 0.0  # per share
         self.last_sale_price = 0.0
         self.last_sale_date = None
 
+        # for backwards compatibility
+        self.sid = asset
+
     def __repr__(self):
         return "Position({0})".format(self.__dict__)
 
diff --git a/zipline/utils/factory.py b/zipline/utils/factory.py
index 805213f6..a529ed06 100644
--- a/zipline/utils/factory.py
+++ b/zipline/utils/factory.py
@@ -21,6 +21,8 @@ import pandas as pd
 import numpy as np
 from datetime import timedelta, datetime
 
+from zipline.assets import Asset
+from zipline.finance.transaction import Transaction
 from zipline.protocol import Event, DATASOURCE_TYPE
 from zipline.sources import SpecificEquityTrades
 from zipline.finance.trading import SimulationParameters
@@ -30,7 +32,7 @@ from zipline.data.loader import (  # For backwards compatibility
     load_bars_from_yahoo,
 )
 from zipline.utils.calendars import get_calendar
-
+from zipline.utils.input_validation import expect_types
 
 __all__ = ['load_from_yahoo', 'load_bars_from_yahoo']
 
@@ -150,27 +152,27 @@ def create_split(sid, ratio, date):
     })
 
 
-def create_txn(sid, price, amount, datetime):
-    txn = Event({
-        'sid': sid,
-        'amount': amount,
-        'dt': datetime,
-        'price': price,
-        'type': DATASOURCE_TYPE.TRANSACTION,
-        'source_id': 'MockTransactionSource'
-    })
-    return txn
+@expect_types(asset=Asset)
+def create_txn(asset, price, amount, datetime, order_id):
+    return Transaction(
+        asset=asset,
+        price=price,
+        amount=amount,
+        dt=datetime,
+        order_id=order_id,
+    )
 
 
-def create_txn_history(sid, priceList, amtList, interval, sim_params,
+@expect_types(asset=Asset)
+def create_txn_history(asset, priceList, amtList, interval, sim_params,
                        trading_calendar):
     txns = []
     current = sim_params.first_open
 
     for price, amount in zip(priceList, amtList):
-        current = get_next_trading_dt(current, interval, trading_calendar)
+        dt = get_next_trading_dt(current, interval, trading_calendar)
 
-        txns.append(create_txn(sid, price, amount, current))
+        txns.append(create_txn(asset, price, amount, dt, None))
         current = current + interval
     return txns