Researchers have recently called for a greater empha sis on the use of functional assessment procedures in the selection of treatment strategies for problem behavior in persons with mental retardation. In applied settings there are several methods available for conducting a functional assessment. This article identifies and de scribes three methods that have been used in applied behavior analysis research: informant assessment (e.g., behavioral interviews, rating scales, and questionnaires), direct observation assessment, and experimental analy sis. Although experimental analysis provides the most conclusive information regarding controlling variables for a behavior problem, less rigorous methods of assess ment also yield important information with which to select and evaluate treatment strategies. This article reviews the strengths and weaknesses of each functional assessment method and provides a case example to illustrate the use of the various assessment procedures.DESCRIPTORS: applied behavior analysis, behav ioral assessment, behavior management, excess behav ior, functional analysis, functional assessment, inter viewing, A-B-C assessmentThe treatment of problem behavior in children and adults with mental retardation has received considera ble attention over the last 20 years. There is an abun dance of treatment research available for practitioners to consider when making treatment decisions (e.g.