Health care simulation includes a variety of educational techniques used to complement actual patient experiences with realistic yet artificial exercises. This field is rapidly growing and is widely used in emergency medicine (EM) graduate medical education (GME) programs. We describe the state of simulation in EM resident education, including its role in learning and assessment. The use of medical simulation in GME is increasing for a number of reasons, including the limitations of the 80-hour resident work week, patient dissatisfaction with being "practiced on," a greater emphasis on patient safety, and the importance of early acquisition of complex clinical skills. Simulation-based assessment (SBA) is advancing to the point where it can revolutionize the way clinical competence is assessed in residency training programs. This article also discusses the design of simulation centers and the resources available for developing simulation programs in graduate EM education. The level of interest in these resources is evident by the numerous national EM organizations with internal working groups focusing on simulation. In the future, the health care system will likely follow the example of the airline industry, nuclear power plants, and the military, making rigorous simulation-based training and evaluation a routine part of education and practice.