Impairments in language and communication are among the core symptoms of autism spectrum disorder (ASD). These impairments are often the first symptoms to emerge and parents may raise initial concerns about their baby, later diagnosed with ASD, because he or she fails to communicate and has not begun speaking. In this talk data is presented from two large-scale longitudinal studies that focus on the early development of language in this population: a study of infants at risk for ASD and a study of toddlers with ASD who were followed from age 2 to 4, with an emphasis on which factors predict variability in language outcomes in ASD. These studies highlight some of the behavioral, environmental, and neurobiological origins of language and communicative development in very young children with ASD. (#25988)