Simulation-based training methods have been widely adopted in hazardous professions such as aviation, nuclear power, and the military. Their use in medicine has been accelerating lately, fueled by the public's concerns over medical errors as well as new Accreditation Council for Graduate Medical Education requirements for outcome-based and proficiency-based assessment methods. This article reviews the rationale for simulator-based training, types of simulators, their historical development and validity testing, and some results to date in laparoscopic surgery and endoscopic procedures. A number of companies have developed endovascular simulators for interventional radiologic procedures; although they cannot as yet replicate the experience of performing cases in real patients, they promise to play an increasingly important role in procedural training in the future.