We argue for the utility of a functional definition of stereotypy based on evidence of both sensory automatic and socially mediated reinforcement contingencies in the occurrence of stereotypy in children with autism. A predetermined sensory function of stereotypy is often invoked in the behavioral literature and the term "self-stimulatory behavior" is commonly misused as interchangeable with "stereotypy." We discuss evidence for a variety of potential functional properties of stereotypy. Diagnostic definitions are reviewed and support for both sensory and social functions is outlined. We argue that stereotypies should be described and categorized according to their function, rather than form. Furthermore, treatment decisions should be based on a functional interpretation of stereotypy, which acknowledges its operant and heterogeneous quality in autism.