the concept of function is central to the discipline of behavior analysis; it serves to characterize the subject matter of the science and is also used to distinguish behavior analysis from other approaches in psychology. in this commentary we assess the concept of function as it is used within behavior analysis. this is done through the perspective of interbehaviorism, with attention to implications for the validity and significance of behavior analysis as a scientific system. problems are identified, and an alternative is proposed. finally, the implications of the adoption of this alternative are reviewed.