This paper chronicles the psychophysical principles which led to the development of the Ratings of Perceived Exertion (RPE) scale by Borg in 1970 and a concise, comprehensive summary of research on the scale. The current status of research is examined by discussion of several important areas within the field including psychophysical scaling, scale development, psychometrics, and applications. Physiological and psychological approaches are examined and the need for an interdisciplinary approach is addressed.