This state-of-the-art paper is one of several volumes which describe the characteristics, quality, and costs of services to severely mentally retarded, severely emotionally disturbed, deaf-blind, and severely multiply handicapped clients age 21 and under. Part I of the volume consists of a subjective distillation of the concepts and research reported in Part II. It discusses many of the issues for which documentation and research support are later provided. It also includes an historical context for the three major categories of handicap and treats some issues on which little research is available. The final section of the first part presents some of the speculations as to the direction that provision of care for the severely handicapped will be taking in the future. Part II is the research review containing five sections. The first four represent the handicapping conditions of interest to this study: severe mental retardation, severe emotional disturbance, deaf-blindness, and severe multiple handicaps. The research is discussed under the same basic rubrics wherever possible. For the three major handicap categories (mental retardation, emotional disturbance, and deaf-blindness), there are the following subsections: problems of definition and classification; prevalence studies; research and demonstration; measurement; guides to providers; and bibliographies. The treatment of the fourth category, multiple handicaps, is much shorter and has fewer subsections. The fifth and last section covers research studies which are concerned with more than one type of handicap. (RC)