In this paper, we illustrate a model-based approach to Web server performance evaluation, and present an analytic queueing model of Web servers in distributed environments. Performance predictions from the analytic model match well with the performance observed from simulation. The model forms an excellent basis for a decision support tool to allow system architects to predict the behavior of new systems prior to deployment, or existing systems under new workload scenarios. Keywords HTTP, Web, distributed, OO, Java, servlet, script, performance.