This chapter begins with a definition of “simulation” and outlines simulation attributes. It then discusses the purpose of simulations, distinguishing and illustrating their various categories and forms in medical and health professional education, and describes their benefits, limitations, and ways to use them effectively. The elements of effective simulations for learning, and why these are important, are then explained. To illustrate these concepts, the chapter concludes by describing health-related simulations developed in the SAGE for Learning project, including COMPS, a collaborative online multimedia problem-based simulation; COMPSoft, a software environment for creating cases and allowing learners to work through them online; HealthSimNet, a simulation for HIV/AIDS patients and professionals to experience navigating the health care system; and MIRAGE, a psychiatry prototype for medical students.