This paper describes the Use Case Simulator (UCSIM), an Eclipse plug-in for visualization and simulation of use cases. Simulation of use cases allows stakeholders to play with what-if scenarios as a way to test completeness and correctness. Textual use cases, however, are not semantically precise enough to be simulated. UCSIM, therefore, uses minimal extensions to UML models to describe use cases, transforms the use cases automatically into an executable form (hierarchical UML state machines), and provides a configurable simulator for executing the state machine representation of the use cases.