This chapter is a review of research on language development in individuals with Down syndrome. The development of various aspects of language is described from infancy to adulthood, with a focus on gestures and prelinguistic communication, speech, vocabulary, syntax, and pragmatics. In each area, the profile of relative strengths and weaknesses is described, as well as the trajectory of development, and differences relative to other disorders, and the influence of other aspects of the phenotype of individuals with Down syndrome (e.g., memory impairments). Research is briefly reviewed on possible influences of the social and linguistic environments of individuals with Down syndrome on their language development. The chapter focuses on research on the learning of English as a first language; however, where possible, data from other languages and cultures is considered. Directions for future research are outlined.

Typically-developing (TD) infants as young as 24 months of age use phonological information to establish links between the words of their early lexicons (Mani & Plunkett, 2010; 2011), which facilitates word recognition and learning. However, Down syndrome (DS) children are reported to have difficulties in learning phonological representations (Jarrold & Thorn & Stephens, 2009). The present study aimed to evaluate if DS children establish lexical networks based on phonological similarity by exploring the effects of lexical competition in a phonological priming task. We evaluated 24 children with DS (mental verbal age; M= 40 months) and 24 children with TD (mental verbal age; M= 40 months), matched by receptive vocabulary size, with a phonological priming adaptation of the intermodal preferential looking task. Children with DS showed inhibition of target recognition in related trials compared to unrelated trials. Children with TD showed an absence of priming effects. Further analysis revealed a relationship between the prime cohort size and the level of inhibition of target recognition for both groups. Our results suggest children with DS possess sufficiently detailed phonological representations that allow them to efficiently cluster their lexical entries based on phonological similarities. Moreover, results are thought to reflect differences in the lexical competition processes between the TD and DS groups.