GIST (Graphical Input Simulation Tool) is a powerful and user-friendly performance evaluation tool for the specification and simulation of Extended Queueing Network models of computer and other systems. It includes a number of modeling abstractions for active and passive resource management, job creation and destruction, synchronization, routing, and statistics collection. Two interfaces are provided: a graphical interface (GUIDE) that allows the user to describe much of the simulation model pictorially; and a textual interface (TIDE) that has the same modeling capabilities but uses a menu-driven, window-oriented approach for use on non-graphics terminals. Object and run-time parameters in both interfaces are specified through menus that reduce the probability of specification-time errors. A number of extensions planned for GIST are also described.