In this chapter, the significance of Modeling & Simulation (M&S) will be discussed. This includes definitions and uses of M&S. There are processes for developing models and simulations. The modeling and simulation process is described and illustrated using the example of traffic flow through a gas station. After the M&S process has been introduced, different simulation approaches are discussed. The two most common approaches are called discrete-event and continuous. These approaches refer to the way in which time is represented in systems. Additional methodologies that are commonly used to represent system behavior are agent-based simulation and system dynamics. Agent-based simulation and system dynamics are introduced through examples. Monte Carlo, which involves random number generation, is then described as a simulation methodology. Monte Carlo simulations are also used as an approach to better understand the role of randomness in behavior of real-life engineering systems.