This article reviews the research on speech and language in children and adolescents with Down syndrome from a practical point of view. It identifies the typical profile of speech and language development, emphasising the variability in development for different individuals, and describes the main reasons for this profile as far as they are understood at the present time. Drawing on this information and what is known about the processes of speech and language development in typically developing children, the paper sets out principles to guide parents, teachers and speech and language therapists as they interact with the children in their care. The main difficulties experienced by children with Down syndrome can be grouped under several headings; difficulties in hearing, auditory perception and processing, difficulties with clear speech production and greater difficulty in learning grammar than vocabulary. These, in turn, are likely to effect the quality and quantity of the language learning opportunities available to the children. Babies and children with Down syndrome need more, high quality learning opportunities in order for them to learn and remember the meanings of words and sentences, yet they get less opportunities because of their slower progress. The author argues that most children and young people with Down syndrome could be helped to improve their speech and language skills if we simply applied the knowledge that we now have more effectively.