This document provides an introduction to simulation. It defines simulation as modeling a real system and experimenting with that model to understand the system's behavior or evaluate different operational strategies. Simulation allows modeling complex systems in detail, describing behavior, constructing theories, predicting future behavior from changes, and analyzing proposed systems. Almost anything can be simulated, with applications including computer systems, manufacturing, business, government, and more. The main advantage of simulation is its ability to answer "what if" questions without disrupting real systems and to test proposed systems. Drawbacks include the challenges of building accurate models and interpreting results, as well as the time and expense of simulation analysis.