Iwata, Dorsey, Slifer, Bauman, and Richman (1982) presented the first comprehensive and standardized methodology for identifying operant functions of aberrant behavior. This essay discusses the significance functional analysis has had for applied behavior analysis. The methodology has lessened the field's reliance on default technologies and promoted analysis of environment-behavior interactions maintaining target responses as the basis for selecting treatments. It has also contributed to the integration of basic and applied research. Future directions for this research are suggested.