This project's past work on longitudinal language development in children and adolescents with Down syndrome has confirmed the existence of a specific expressive language deficit, increasing with age, that affects syntax most severely, and additional delays in syntax comprehension emerging in adolescence. It has also demonstrated that language acquisition does not stop with the onset of adolescence or the acquisition of simple syntax, offering the possibility of effective language learning in teenage years. Auditory short-term memory deficits, of severity equal to the expressive syntax deficit were also present, and predictive of fast-mapping of novel words in production. The purpose of the current project is to study short-term, microgenetic processes of language acquisition, comprehension, and use in 30 teenagers (13 to 18 years) with Down syndrome within a theoretical framework (the Child Talk model) that emphasizes the role that prior history of talk in context plays in making language available, a history attenuated by auditory short-term memory deficits in children with Down syndrome. Our planned studies will a) evaluate the role of auditory short-term memory support and examiner sentence repetition in incidental learning ("fast mapping") of novel words and in qualitative shifts of sentence comprehension strategies; b) examine utterance formulation, turn-taking, and latency patterns in an interview format; and c) assess the discourse contexts and acquisition of new discourse structures through repeated and expanded prompting of story telling in microgenetic studies that follow acquisition across 5 or 6 sessions. Performance will be compared to 2 control groups matched on a) age and noverbal IQ and b) typically developing children matched on mean length of utterance in narrative or syntactic comprehesion. The role of auditory short-term memory in these studies will be evaluated by the Nonword Repetition Task (or Digit Span if unscorable due to intelligibility constraints), with ANCOVAs carried out to evaluate its contribution to effects. The final study will evaluate the phenotypic specificity of discrepancy patterns in language and cognitive status measures through discriminant function analysis of the group with Down syndrome and a group with cognitive disability of unknown origin.