Publisher Summary The growing Internet/Web infrastructure has placed increasingly larger demands on servers as well as the networks connecting them. In particular, a crowded Internet can slow the delivery of responses (latency) to customer queries and transactions to a crawl, resulting in poor quality of service (QoS). The result is disappointed users and loss of business. Thus, performance evaluation of distributed client/server and web-based applications has become extremely important. A capacity planning is the activity of predicting system performance for a given set of resource requirements to achieve a desired level of performance. A capacity planning activity would launch a performance evaluation study via an analytical or simulation model to predict certain statistics of response times, and unearth any “performance bottlenecks” in the system. The chapter focuses on simulation modeling of computer-based information systems, especially those configured in a client/server architecture. A number of examples illustrate capacity planning for such systems.