Should conduct disorder (CD) be classified as a psychiatric diagnosis? The model of CD found in current diagnostic systems is not without its flaws. The criteria for CD have often been criticised for lacking developmental sensitivity with regard to young children, and questions concerning the subtyping of the disorder and its overlap with comorbid conditions remain contentious. Compelling evidence nonetheless supports the view that this ‘behaviour disorder’ represents a complex mental health issue that belongs in these diagnostic systems. Most importantly, it identifies a group of children and adolescents for whom early intervention appears to be particularly critical and may serve to divert chronic trajectories of poor mental health. This debate has significant implications for science and practice and is addressed here with a particular focus on the Australian context.