Discrete event simulation performance modeling tools have been used by systems engineers and technical architects for a number of years to analyze the behavior of a variety of types of systems. Some specialized performance modeling tools have been developed to analyze client/server enterprise systems. Technical architects use these tools to model the behavior of systems with alternative configurations to simulate the flow of transactions through a system over time. The tools provide statistical and other means to alter the interarrival times of transactions to better simulate variations in system behavior due to changes in transactions on a seasonal, daily, or other basis. Performance modeling helps identify bottlenecks, or problems with performance, in systems. Through simulated improvements in system capacities, the model developer can alter the configuration to better handle the projected levels of transactions. The paper presents an introduction to discrete event simulation performance modeling tools and discusses their application to enterprise GIS systems design. Federal agency case studies of enterprise GIS systems design using performance modeling are also presented.