In analyzing the performance of computer and digital communication systems, contention for finite capacity resources is often seen to be a dominant factor. Extended Queueing Network (EQN) models are appropriate for modeling suck systems and bave been used with considerable success, EQN models can be solved by exact analysis, by approximate analysis, or by simulation. This thesis is concerned with the design of a high level tool for solving EQN models by simulation that accepts specifications in a natural graphical manner. The primary motivation for this research was to prove the feasibility of providing a versatile tool that is easy to learn and use and is complete with respect to EQN models. Existing software tools for EQN modeling do not take advantage of the fact that a natural way to specify such models is graphical. Our modeling tool, the Graphical Input Simulation Tool (GIST), achieves these objectives 1) by utilizing a transaction-oriented approach as opposed to a language-based approach, 2) by providing two user interfaces, a graphical interface and a textual interface, that permit specification of EQN models at a very high level of abstraction, and 3) by means of a versatile set of modeling abstractions. In terms of modeling capabilities, GIST provides analogs of most abstractions commonly found in other high-level BQN modeling tools and also includes abstractions that have no counterparts in other tools. Ve demonstrate the feasibility and utility of providing a GISTlike tool and conclude that the transaction oriented-approach is also applicable and appropriate for building modeling tools in areas other than EQNs. GIST can also be extended to further research in performance evaluation tools and in performance evaluation in general.