This article describes the role of a medical evaluation in the overall developmental assessment of a young child. General guidelines for conducting the evaluation and practical issues to consider in dealing with young children and their families are provided. To illustrate these points, hypotonia and failure to thrive, two broad categories of pathology with numerous potential underlying medical causes, are addressed specifically. The involvement of genetics in developmental pathology is also outlined in some detail. Finally, issues to be aware of when considering pharmacologic management of psychiatric symptoms in young children are discussed.