The aims of this longitudinal study were: (1) to assess the continuity and change in diagnosis, intelligence, and language skills in children with autism, Down syndrome, and other developmental delays, (2) to specify the deficits in social competence and language skills in these children, and (3) to identify precursors in the preschool period of gains in language skills and of peer engagement in the mid-school years. The initial sample consisted of 70 children with autism, 93 children with Down syndrome, 59 children with developmental delays, and 108 typically developing children, with the first three groups of children studied when they were between 2 and 6 years of age. At follow-up, 51 children with autism, 71 children with Down syndrome, and 33 children with developmental delays were assessed at mean ages around 10-13 years. The long-term follow-up showed little change in the diagnosis of autism but sizeable improvements in intellectual and language abilities within the autistic group, a pattern that was not seen in the children with Down syndrome. Unique deficits in joint attention, some forms of representational play, responsiveness to the emotions of others, and initiation of peer engagement were identified in the autistic children, whereas the children with Down syndrome seemed to have a specific deficit only in language. Joint attention skills were concurrently associated with language abilities in all groups and predicted long-term gains in expressive language for the children with autism. Children with autism, regardless of their level of functioning, were less socially engaged with classmates than the other developmentally disabled children because they infrequently initiated and accepted play bids, not because they were rebuffed by peers. Early nonverbal communication and play skills were predictors of the frequency of initiations of peer play for the children with Down syndrome as well as the extent of peer engagement of the children with autism. These results suggest that improvements in early communication and play skills may have long-term consequences for later language and social competence in these groups of children.