This is the second in a series introducing the activity model in the Unified Modeling Language, version 2 (UML 2), and how it integrates with the action model . The first article covered motivation and architecture for the new models, basic aspects of UML 2 activities and actions, and introduced the general notion of behavior in UML 2 . The remainder of the series elaborates specific elements in the model. This article recaps behavior models in UML and the role of actions in them. It covers the execution characteristics of actions in general, which inherit to the many kinds of actions provided in UML 2. It also covers additional characteristics of actions that invoke behaviors.