Both simulation and analytic models of computer systems can be very useful for predicting the performance of proposed new systems or proposed changes to exmting systems. Unfortunately, many potential users of models are reluctant to use them because of the complemty of many such models and the difficulty of relating the model to the real system This tutorial paper leads the reader through the development and use of an easily understood analytic model. This is then placed m context with a class of similar analytic models. In spite of the simplicity of these models they have proved useful and quite accurate in predicting performance (utilization, throughput, and response) using only the most basic system data as input These parameters can either be estimates or measurements from a running system. The model equations and assumptions are defined, and a detailed case study is presented as an example of their use