The historical roots of simulation might be described with the broadest definition of medical simulation: "an imitation of some real thing, state of affairs, or process" for the practice of skills, problem solving, and judgment. From the first "blue box" flight simulator to the military's impetus in the transfer of modeling and simulation technology to medicine, worldwide acceptance of simulation training is growing. Large collaborative simulation centers support the expectation of increases in multidisciplinary, interprofessional, and multimodal simulation training. Virtual worlds, both immersive and Web-based, are at the frontier of innovation in medical education.