Abstract: A graphical, interactive debugger for a computer-based discrete-event simulation model of systems having parallel processes includes a run-time graphic user interface module in which predefined events of the executing simulation model are animated, and a run-time interface simulator module for controlling execution of the simulation model based on commands from the graphical user interface. The model and the animation are based on hierarchical directed process execution graphs in which a transaction represents one of several processes executing in parallel within the system modeled with the graphs. One transaction at a time is shown moving between nodes in one of the graphs. The animation follows the transaction as it moves between sub-models represented by each graph, switching to other graphs as needed. Once the transaction is blocked, the next transaction is displayed in context of the appropriate graph and followed until it is blocked. The screen on which animation is presented is divided into two areas.