Capacity planning is a powerful tool for managing quality of service on the Web. This tutorial presents a capacity planning methodology for Web-based environments, where the main steps are: understanding the environment, characterizing the workload, modeling the workload, validating and calibrating the models, forecasting the workload, predicting the performance, analyzing the cost-performance plans, and suggesting actions. The main steps are based on two models: a workload model and a performance model. The first model results from understanding and characterizing the workload and the second from a quantitative description of the system behavior. Instead of relying on intuition, ad hoc procedures and rules of thumb to understand and analyze the behavior of Web services, this tutorial emphasizes the role of models, as a uniform and formal way of dealing with capacity planning problems.