Evaluating the performance of a system is of central concern in making engineering decisions. When direct measurement of performance is not possible or feasible, evaluation consists of two phases: specification of an appropriate performance model, and evaluation of the model to obtain the performance measures. Broadly, a performance model can be evaluated by exact or approximate analysis, or by simulation. A class of models popular for evaluation of a number of systems, computer systems in particular, is that of Extended Queuing Network (EQN) Models. Software tools are typically used for building EQN models for evaluation through analyses or simulation. This thesis describes an effort in experimenting with an approach to the design and implementation of a tool for performance evaluation of EQN models via simulation. The objective in this effort is to design a tool that is easy and intuitive to use, yet versatile and powerful in its modeling capabilities. The tool we have implemented is called Graphical Input Simulation Tool (GIST). GIST meets its design objectives by (1) providing a pair of user interfaces that are capable of accepting the abstract EQN model specification directly, are easy and intuitive to learn and use, and are helpful in quick model specification with reduced likelihood of semantic and syntactic specification errors, and (2) incorporating into the set of EQN objects it provides, the capabilities perceived necessary for realistic modeling of activities that characterize the systems of interest.