The purpose of this paper is to show how system operations can be specified by pre- and postconditions using the Object Constraint Language (OCL), as defined by the Unified Modeling Language (UML) specification, and how these system operation specifications relate to the analysis class model and other models of the analysis phasis. It is also shown how the communication between the system and actors by event sending can be modeled. The result is a system behavior model at a higher-level of abstraction than the one provided by collaboration diagrams, which concentrate on method invocations between objects.