The development of experimental functional analysis and more recently functional analysis checklists have become common technologies for evaluating antecedent events and the consequences of problematic behaviors. Children and developmentally disabled persons across the life span with challenging behaviors have been the primary focus of this research. The primary purpose of this paper is to present an overview the developments in this rapidly expanding research literature, particularly as it involves the application of the functional assessment paradigm in applied settings where resources and time are scarce. Implication of the functional assessment research for clinical practice are discussed along with strengths and weakness of the current technology.