In this chapter, we start by looking at some basic quantitative relationships, which can be used to evaluate a system’s performance based on measured or known data, a process known as operational analysis. In the second part, we provide a brief introduction to the basic notation and principles of queueing theory. While queueing theory has been applied successfully to different domains, for example, to model manufacturing lines or call center operation, in this chapter, we focus on using queueing theory for performance evaluation of computer systems. The chapter is wrapped up with a case study, showing in a step-by-step fashion how to build a queueing model of a distributed software system and use it to predict the performance of the system for different workload and configuration scenarios.