diff --git a/graphkit/network.py b/graphkit/network.py
index 24c3ac37..04f0dca1 100644
--- a/graphkit/network.py
+++ b/graphkit/network.py
@@ -8,6 +8,7 @@
 from io import StringIO
 
 from .base import Operation
+from .modifiers import optional
 
 
 class DataPlaceholderNode(str):
@@ -141,6 +142,65 @@ def compile(self):
                 raise TypeError("Unrecognized network graph node")
 
 
+    def _collect_satisfiable_needs(self, operation, inputs, satisfiables, visited):
+        """
+        Recusrively check if operation inputs are given/calculated (satisfied), or not.
+
+        :param satisfiables:
+            the set to populate with satisfiable operations
+
+        :param visited:
+            a cache of operations & needs, not to visit them again
+        :return:
+            true if opearation is satisfiable
+        """
+        assert isinstance(operation, Operation), (
+            "Expected Operation, got:",
+            type(operation),
+        )
+
+        if operation in visited:
+            return visited[operation]
+
+
+        def is_need_satisfiable(need):
+            if need in visited:
+                return visited[need]
+
+            if need in inputs:
+                satisfied = True
+            else:
+                need_providers = list(self.graph.predecessors(need))
+                satisfied = bool(need_providers) and any(
+                    self._collect_satisfiable_needs(op, inputs, satisfiables, visited)
+                    for op in need_providers
+                )
+            visited[need] = satisfied
+
+            return satisfied
+
+        satisfied = all(
+            is_need_satisfiable(need)
+            for need in operation.needs
+            if not isinstance(need, optional)
+        )
+        if satisfied:
+            satisfiables.add(operation)
+        visited[operation] = satisfied
+
+        return satisfied
+
+
+    def _collect_satisfiable_operations(self, nodes, inputs):
+        satisfiables = set()
+        visited = {}
+        for node in nodes:
+            if node not in visited and isinstance(node, Operation):
+                self._collect_satisfiable_needs(node, inputs, satisfiables, visited)
+
+        return satisfiables
+
+
     def _find_necessary_steps(self, outputs, inputs):
         """
         Determines what graph steps need to pe run to get to the requested
@@ -204,6 +264,13 @@ def _find_necessary_steps(self, outputs, inputs):
             # Get rid of the unnecessary nodes from the set of necessary ones.
             necessary_nodes -= unnecessary_nodes
 
+        # Drop (un-satifiable) operations with partial inputs.
+        # See https://github.com/yahoo/graphkit/pull/18
+        #
+        satisfiables = self._collect_satisfiable_operations(necessary_nodes, inputs)
+        for node in list(necessary_nodes):
+            if isinstance(node, Operation) and node not in satisfiables:
+                necessary_nodes.remove(node)
 
         necessary_steps = [step for step in self.steps if step in necessary_nodes]