Functional assessment has significantly improved the success of behavioral treatment of problem behaviors in adults with mental retardation. Functional assessment methods (i.e., techniques that yield a hypothesis of functional relationships) include direct observation, interviews, and checklists. Functional analysis consists of empirical methods that demonstrate behavioral function in controlled settings. Each method has advantages and limitations that reflect differences in both available resources and individual client characteristics. These methods and issues are reviewed, and future directions for both the research field and the adult population are suggested.