This chapter describes mechanisms by which we may observe and characterize the performance of systems. It concentrates on the wide variety of systems whose elements include waiting lines (queues), and studies how such waiting lines interact with other activities or entities in the system toward achieving certain goals for system throughput. A queueing system consists of a flow of customers into and through a system, who are to be processed or dealt with by one or more servers. An exponential interarrival distribution implies that the arrival process is Poisson distributed. If the interarrival times are exponential, then the number of arrivals per unit time is a Poisson process. To facilitate our development of formulae for performance analysis of queueing systems in steady-state, the chapter illustrates the system by using a particular type of transition diagram known as a birth-and-death process model. Application tools for queueing and simulation studies are abundant.