Publisher Summary This chapter discusses the history of psychological theory and research in mental retardation since World War II. The concepts of mental retardation are quite ancient, modern scientific psychology, including its concern with mental retardation, developed only in the late 19th century. There was a time when the main clinical activity of psychologists working in mental retardation was administering intelligence tests. Their work in counseling and therapy with retarded persons and their families was not supported by research evidence. The development of applied behavior analysis, such as in the treatment of psychopathology, including aggression, self-injurious behavior, stereotypies, and autism, as well as self-help and communicative skills and in persons with retardation, spearheaded change. The use of psychotropic medications in institutions for persons with retardation is discussed in this chapter. Phenothiazines, such as chlorpromazine, had first been introduced in France in the early 1950s and were widely used to treat patients with schizophrenia. The study of psychopathology in mental retardation has moved from a state of ignorance, regarding the relationship of these two types of disorders, to the realization that they overlap substantially. Interventions, both behavioral and pharmacological, have been developed and studied rigorously.