This article presents principles and basic steps for practitioners to complete a functional analysis of client behavior. The emphasis is on application of functional analysis to adult mental health clients. The article includes a detailed flow chart containing all major functional diagnoses and behavioral interventions, with functional assessment questions directing the reader through the chart. The model presented incorporates both operant and classical conditioning and a strategy for selecting target behaviors consistent with behavioral principles. Finally, the article describes the continuing role of functional analysis in effective behavioral intervention and its potential advantages over other forms of assessment in contemporary cognitive-behavioral therapy. (Contains 1 table and 9 figures.)