Children with Down syndrome face considerable intellectual challenges. Despite the widespread belief that their social understanding is relatively "spared," many also experience significant difficulties at an interpersonal level. This chapter assesses the literature on sociocognitive development in Down syndrome and the evidence for strengths or weaknesses in understanding the social behaviors, emotions, and intentions of others. It highlights the predominant focus to date on infants and preschool children and the consequent need for more research at older ages, ideally through larger-scale, cross-syndromic, and longitudinal studies. It also underlines the need to tie knowledge of developmental trajectories not only to the underlying neurobiology of Down syndrome but also to aspects of the children's social environment. Investigating how interpersonal understanding is used in learning contexts and translating this knowledge into more focused support programs should also be a priority.