Individuals with Down syndrome are predisposed to show a specific behavioural phenotype, or a pattern of strengths and challenges in functioning across different domains of development. It is argued that a developmental approach to researching the Down syndrome behavioural phenotype, including an examination of the dynamic process of the unfolding of the phenotype, will advance science and service for this population. Related issues including the distinction between primary and secondary phenotypic features, heterotypic continuity, and methodological implications are discussed.