The author defines the term standardized patient (SP), the umbrella term for both a simulated patient (a well person trained to simulate a patient's illness in a standardized way) and an actual patient (who is trained to present his or her own illness in a standardized way). He first discusses the many values of simulated patients over actual patients as teaching and assessment tools in the classroom and refutes a few myths about the use of SPs. Then he recounts the origin and development of SPs over a three-decade period, beginning with his work as a neurologist at the Los Angeles County Hospital, where he trained a model from the art department to simulate a neurological patient and assist in the assessment of clinical clerks. He then describes additional roles of SPs that have developed, including: (1) their use in the Clinical Practice Examination created at Southern Illinois University School of Medicine and (2) the major use that has come into being over the last 10-15 years; facilitating the comprehensive assessment of clinical competence using multiple stations in examinations such as the objective structured clinical examination. He concludes with information about recent and current work on SPs, who are becoming more and more accepted in the assessment process, and urges skeptics not to make judgments about the value of SPs until they have experienced the technique firsthand and reviewed the literature concerning the extensive and often high-quality research about this assessment tool.