This article presents a brief historical overview of psychiatric diagnosis in persons with developmental disabilities, including a review of the relationship between applied behavior analysis and dual diagnosis. The purposes of diagnosis are then outlined. The definitions of developmental disabilities, mental disorder, and dual diagnosis are reviewed and some of the problems when making a psychiatric diagnosis in persons with developmental disabilities are discussed. Special attention is given to the problem of diagnosing mental retardation in persons with mild through borderline mental retardation and to the problem of diagnosing psychiatric disorders in persons with severe and profound mental retardation. Finally, an overview of common issues in the process of collecting and combining information to make a psychiatric diagnosis in persons with developmental disabilities is given.