diff --git a/zipline/pipeline/engine.py b/zipline/pipeline/engine.py
index 2fc54c5a..4bb6f6b0 100644
--- a/zipline/pipeline/engine.py
+++ b/zipline/pipeline/engine.py
@@ -344,6 +344,8 @@ class SimplePipelineEngine(object):
         loader_group_key = juxt(get_loader, getitem(graph.extra_rows))
         loader_groups = groupby(loader_group_key, graph.loadable_terms)
 
+        refcounts = graph.initial_refcounts(workspace)
+
         for term in graph.ordered():
             # `term` may have been supplied in `initial_workspace`, and in the
             # future we may pre-compute loadable terms coming from the same
@@ -380,6 +382,11 @@ class SimplePipelineEngine(object):
                 else:
                     assert workspace[term].shape == (mask.shape[0], 1)
 
+                # Decref dependencies of ``term``, and clear any terms whose
+                # refcounts hit 0.
+                for garbage_term in graph.decref_dependencies(term, refcounts):
+                    del workspace[garbage_term]
+
         out = {}
         graph_extra_rows = graph.extra_rows
         for name, term in iteritems(graph.outputs):
diff --git a/zipline/pipeline/graph.py b/zipline/pipeline/graph.py
index 9ff09d45..667c3be9 100644
--- a/zipline/pipeline/graph.py
+++ b/zipline/pipeline/graph.py
@@ -119,6 +119,54 @@ class TermGraph(DiGraph):
     def _repr_png_(self):
         return self.png.data
 
+    def initial_refcounts(self, initial_terms):
+        """
+        Calculate initial refcounts for execution of this graph.
+
+        Parameters
+        ----------
+        initial_terms : iterable[Term]
+            An iterable of terms that were pre-computed before graph execution.
+
+        Each node starts with a refcount equal to its outdegree, and output
+        nodes get one extra reference to ensure that they're still in the graph
+        at the end of execution.
+        """
+        refcounts = self.out_degree()
+        for t in self.outputs.values():
+            refcounts[t] += 1
+
+        for t in initial_terms:
+            self.decref_dependencies(t, refcounts)
+
+        return refcounts
+
+    def decref_dependencies(self, term, refcounts):
+        """
+        Decrement in-edges for ``term`` after computation.
+
+        Parameters
+        ----------
+        term : zipline.pipeline.Term
+            The term whose parents should be decref'ed.
+        refcounts : dict[Term -> int]
+            Dictionary of refcounts.
+
+        Return
+        ------
+        garbage : set[Term]
+            Terms whose refcounts hit zero after decrefing.
+        """
+        garbage = set()
+        # Edges are tuple of (from, to).
+        for parent, _ in self.in_edges([term]):
+            refcounts[parent] -= 1
+            # No one else depends on this term. Remove it from the
+            # workspace to conserve memory.
+            if refcounts[parent] == 0:
+                garbage.add(parent)
+        return garbage
+
 
 class ExecutionPlan(TermGraph):
     """