This paper argues for approaching patient safety through human factors engineering, which studies the day-to-day activities of health care professionals and uses them as a vehicle for changing the health care system. In the authors' framework, the physical, cognitive, and social/behavioral performance of health care professionals is analyzed to develop methods of increasing the likelihood of successful performance. The authors contrast this approach to other patient safety paradigms that seek to reduce errors made by providers or increase the use of evidence-based medicine. This article is part of a special issue on the role of design in health care improvements.