Remember our advice to use verbs in your Use Cases to describe all of the responsibilities that you think an object has for other objects. Some of those verbs may become associations. An association between two classes simply means that objects created by those classes will be passing messages to each other. In the beginning, don't worry about the details of how that will work. Just pick a meaningful label for the association. The programmer can worry about details later when she or he is writing method bodies into the templates that get generated. Stay at a high level at first while you capture the relationships between objects.