An extensive literature on the causes of challenging behaviors has been developed, primarily in the applied behavior analysis literature. One hundred and seventy-three empirical studies were reviewed where functional assessment serves as the primary method of identifying these causes. Most of the studies were able to identify a clear function or functions. Most commonly established causes were attention, the efforts to acquire tangibles, negative reinforcement in the form of escape from tasks or environments, and sensory stimulation, also described as an alone condition. Examples are provided regarding how these conditions are investigated across studies. Biological and cognitive causes have also been demonstrated. However, to date the empirical literature is limited with the bulk of studies being correlational. Considerably more research is needed, but some causes and methods to identify them are beginning to emerge. (Contains 1 table.)