diff --git a/zipline/gens/mavg.py b/zipline/gens/mavg.py
new file mode 100644
index 00000000..54826d23
--- /dev/null
+++ b/zipline/gens/mavg.py
@@ -0,0 +1,97 @@
+
+from numbers import Number
+from datetime import datetime, timedelta
+from collections import defaultdict
+
+from zipline import ndict
+from zipline.gens.transform import EventWindow
+
+class MovingAverage(object):
+    """
+    Class that maintains a dictionary from sids to EventWindows
+    Upon receipt of each message we update the
+    corresponding window and return the calculated average.
+    """
+
+    def __init__(self, delta, fields):
+        self.delta = delta
+        self.fields = fields
+
+        # No way to pass arguments to the defaultdict factory, so we
+        # need to define a method to generate the correct EventWindows.
+        self.sid_windows = defaultdict(self.create_window)
+
+    def create_window(self):
+        """Factory method for self.sid_windows."""
+        return MovingAverageEventWindow(self.delta, self.fields)
+
+    def update(self, event):
+        """
+        Update the event window for this event's sid.  Return an ndict from
+        tracked fields to averages.
+        """
+        assert isinstance(event, ndict),"Bad event in MovingAverage: %s" % event
+        assert event.has_key('sid'), "No sid in MovingAverage: %s" % event
+        assert event.has_key('dt'), "No dt in MovingAverage: %s" % event
+
+        # This will create a new EventWindow if this is the first
+        # message for this sid.
+        window = self.sid_windows[event.sid]
+        window.update(event)
+        return window.get_averages()
+
+class MovingAverageEventWindow(EventWindow):
+    """
+    Calculates a moving average over all specified fields.
+    """
+    def init(self, fields):
+        # Subclass initializer.  Superclass also expects a timedelta.
+        self.fields = fields
+        self.totals = defaultdict(float)
+
+    def handle_add(self, event):
+        # Sanity check on the event.
+        self.assert_all_fields(event)
+        # Increment our running totals with data from the event.
+        for field in self.fields:
+            self.totals[field] += event[field]
+
+    def handle_remove(self, event):
+        # Decrement our running totals with data from the event.
+        for field in self.fields:
+            self.totals[field] -= event[field]
+
+    def average(self, field):
+        """
+        Calculate the average value of our ticks over a given field.
+        """
+        # Sanity check.
+        assert field in self.fields
+
+        # Averages are 0 by convention if we have no ticks.
+        if len(self.ticks) == 0:
+            return 0.0
+        
+        # Calculate and return the average.  len(self.ticks) is O(1).
+        else:
+            return self.totals[field] / len(self.ticks)
+
+    def get_averages(self):
+        """
+        Return an ndict of all our tracked averages.
+        """
+        out = ndict()
+        
+        for field in self.fields:
+            out[field] = self.average(field)
+        return out
+
+    def assert_all_fields(self, event):
+        """
+        We only track events with all the fields we care about.
+        """
+        for field in self.fields:
+            assert event.has_key(field), \
+                "Event missing [%s] in MovingAverageEventWindow" % field
+            assert isinstance(event[field], Number), \
+                "Got %s for %s in MovingAverageEventWindow" % (event[field], field)
diff --git a/zipline/gens/transform.py b/zipline/gens/transform.py
index ee8cd649..249b44f2 100644
--- a/zipline/gens/transform.py
+++ b/zipline/gens/transform.py
@@ -7,6 +7,7 @@ from copy import deepcopy
 from datetime import datetime
 from collections import deque, defaultdict
 from numbers import Number
+from abc import ABCMeta, abstractmethod
 
 from zipline import ndict
 from zipline.gens.utils import assert_sort_unframe_protocol, \