diff --git a/knowledge/README.txt b/knowledge/README.txt
index 0b98e36..b03af6d 100644
--- a/knowledge/README.txt
+++ b/knowledge/README.txt
@@ -49,7 +49,11 @@ programming with Python:
 Now we can ask what knowledge john is lacking if he would like to take
 the position offered:
 
-  >>> john.getMissingKnowledge(pos01) == (ooProg, pyOo,)
+  >>> missing = john.getMissingKnowledge(pos01)
+  >>> missing
+  (<...KnowledgeElement...>, <...KnowledgeElement...>)
+  
+  >>> missing == (ooProg, pyOo,)
   True  
 
 Luckily there are a few elearning content objects out there that
@@ -65,6 +69,10 @@ provide some of the knowledge needed:
 So that we are now able to find out what john has to study in order to
 fulfill the position offered:
 
-  >>> list(john.getProvidersNeeded(pos01))
+  >>> prov = list(john.getProvidersNeeded(pos01))
+  >>> len(prov)
+  2
+  >>> [d[0] for k, d in prov] == [doc01, doc02]
+  True
   
 
diff --git a/knowledge/interfaces.py b/knowledge/interfaces.py
index 531ab4b..ec0c340 100644
--- a/knowledge/interfaces.py
+++ b/knowledge/interfaces.py
@@ -90,10 +90,10 @@ class IKnowing(Interface):
         """
 
     def getProvidersNeeded(position):
-        """ Return two tuples: first a tuple of knowledge providers that
-            provide knowledge needed for fulfilling the position given,
-            second a tuple with the requirements that could not
-            be met by the available knowledge providers.
+        """ Return a tuple of tuples: Each tuple has as its first element
+            a requirement of the position, the second element is a tuple
+            of the knowledge providers providing this knowledge
+            ((requirement, (provider, ...,)), ...).
         """
 
 
diff --git a/knowledge/knowing.py b/knowledge/knowing.py
index 8ef0877..1e21b6e 100644
--- a/knowledge/knowing.py
+++ b/knowledge/knowing.py
@@ -45,8 +45,20 @@ class Knowing(object):
         del obj._knowers[self]
 
     def getMissingKnowledge(self, position):
-        # to be done
-        return tuple(position.getRequirements())
+        knowledge = self.getKnowledge()
+        missing = []
+        toCheck = [k for k in position.getRequirements() if k not in knowledge]
+        while toCheck:
+            k = toCheck.pop()
+            missing.insert(0, k)
+            for d in k.getDependencies():
+                if d in knowledge or k in toCheck:
+                    continue
+                if d in missing:
+                    # TODO: rearrange missing, but care for cycles...
+                    continue
+                toCheck.append(d)
+        return tuple(missing)
 
     def getProvidersNeeded(self, position):
         return ((k, k.getProviders())