Of course, my previous post about fixing the Factory should not be taken to mean that I actually understand why it works now. In fact, quite the opposite. Emboldened by this small step forward, I tried creating a new type of ThingProduct, with more or less the same results as before: Can't add the new type of Thing to the Place. This time, the factory isn't broke (which was probably something I did trying to fix the original problem). Auuuughhhhh! ^_^ I really think the following page is doing more to confuse me than to help me ;-) http://www.zope.org/Documentation/Guides/ZDG-HTML/ZDG.6.3.7.html