Among many of Jack Michael's contributions to the field of behavior analysis is his behavioral account of motivation. This paper focuses on the concept of "motivating operation" (MO) by outlining its development from Skinner's (1938) notion of "drive." Conceptually, Michael's term helped us change our focus on how to study motivation by shifting its origins from the organism to the environment. Michael's account also served to stimulate applied research and to better understand behavioral function in clinical practice.