Simulation is the process of studying the behavior of an existing or proposed system by observing the behavior of a model representing the system. This chapter introduces simulation models and describes how they can be used in analyzing and predicting the performance of systems under varying circumstances. It presents some guidelines that should be helpful in developing the ability to understand and build simulation models. Simulations can be categorized as either discrete or continuous. This distinction refers to the variables that describe the state of the system. The system state is defined by the set of entities and attributes, and the state of the system typically changes as time progresses. Processes that affect the system state are called activities. Simulation studies can be facilitated by a wide variety of software packages and languages. Specialized computer programming languages have been introduced over the past several decades to assist simulation analysts in the development and use of simulations of real systems.