This paper presents a discussion of the integration of handicapped children into preschool programs. The paper is divided into five sections. The first section defines some of the terms frequently used in discussions of mainstreaming. These terms include mainstreaming; normalization; P. L. 94-142, the law providing for education for all handicapped children; and IEP, the Individualized Education Plan. The second section presents a discussion of both the legal and legislative arguments used in favor of mainstreaming and the potential benefits of mainstreaming for handicapped and nonhandicapped children. The third section reviews research related to social interactions in integrated preschool settings. This review includes research on spontaneous interactions, the facilitation of interactions, and recommendations for future research in this area. The fourth section of the paper describes preschool programs which have successfully integrated handicapped and nonhandicapped children. The programs described include center- and research-based programs, transitional programs, and open education approaches. The final section of the paper discusses some of the issues involved in preschool mainstreaming. This discussion focuses on the training of personnel, the identification of the components of successful programs, the mainstreaming of severely handicapped children, the arrangement of the environment, and criteria for the evaluation of integrated programs. (BD)