This monograph describes the program of the Language and Cognitive Development Center (Massachusetts), which serves toddlers and school-aged children with autism or other pervasive developmental disorders (PDD). An introduction presents incidence figures, the program's philosophy, the program's approach to assessment, intervention with children having limited reality systems, approaches to generalization of training, dealing with behavior problems, and the program's unique approach to teaching language. Next, a discussion of cognitive-developmental systems theory examines child development in terms of the sign stage of development (0-18 months), formation and dynamics of reality systems during the sign stage, and the development of intention during the sign stage. Intervention is then considered, including the program's "Umwelt" approach to the evaluation of reality systems, correcting developmental dysfunctions, developing body schema, coping with surroundings, developing social contact, and developing communication and representation skills. The integration of skills learned in the Center into the home environment is then explained. A final discussion answers questions concerning the curriculum for school-age nonverbal or limited verbal PDD children, examines determinants of child success, and offers evidence of the program's success (48 percent of Center children have returned to the public school mainstream for all or part of their classes). (Contains 53 references.) (DB)