This guide to mainstreaming mentally retarded preschoolers is one of a series of eight manuals on mainstreaming preschoolers developed by Project Head Start. The guide is addressed to parents, teachers and other professionals and paraprofessionals. Chapter I presents information on the meaning, benefits and implementation of mainstreaming and discusses the role of the teacher. Chapter II defines mental retardation, focusing on levels of retardation, rate of learning, commonly associated handicaps and diagnostic problems. An observational checklist and general guidelines for identifying children for referral are included. Chapter III discusses how mental retardation affects learning and development in 3- to 5-year-olds and describes children with mild, moderate and severe handicaps. Chapter IV focuses on planning, physical setting, and teaching techniques for mainstreamed mentally retarded children. Chapter V discusses parent/teacher cooperation, while Chapters VI and VII provide information on specialists, organizations and other sources of help. Notes on screening and diagnosis are appended. (CM)