Methods and outcomes from functional behavioral assessment have been researched widely over the past twenty-five years. However, several important research questions have yet to be examined sufficiently. This quantitative review of developmental disability research aims to make comparisons of different functional behavioral assessment methodologies, both across and within diagnostic categories. Quantitative synthesis data were used to answer questions regarding behavioral function, assessment type, differences based upon diagnostic category, and treatment effectiveness. Results indicate that assessment methodology does not impact treatment effectiveness, but both identified functions and treatment effectiveness are impacted by diagnosis. Implications for clinicians as well as future research directions are also discussed. (Contains 7 tables.)