Discrete event simulation and agent-based modeling are the subjects of this book. These types of simulation are merely two of many with others including systems dynamics, finite element analysis (FEM), and physical, human simulation. This latter type can involve running actual people through a scenario or game. Yet, discrete event simulation and agent-based modeling can offer natural approaches to help people think introspectively about their systems and realize efficiency gains. An animated version of the system being modeled is often a major outcome of the modeling activity. These animations can provide the best available way to engage untrained people in the application of all types of operations research and systems improvement activities.