The study of behavioural phenotypes and their clinical relevance diagnostically and therapeutically is receiving increasing attention. A clear understanding of their nature, aetiology and implications by child mental health professionals is essential if they are to be of real benefit to clients with such problems and their families. This article addresses these issues and considers historical aspects as well as contrasting clinical approaches to the interface between genetics and child mental health. Important examples of conditions with associated behavioural phenotypes are reviewed followed by a discussion of the importance of diagnosis and the roles and responsibilities of child mental health professionals.