This chapter focuses on the clinical phenomenology and descriptive classification of unipolar disorders of childhood. The advent of reliable and valid measures of present mental state in children and adolescents has greatly advanced our understanding of major (unipolar) depression. Research confirms that the clinical picture of affective disorders in children and adolescents resembles the presentation of that in adults. Comorbidity is the concurrent presence of two or more disorders greater than expectation by chance alone. The first way of defining depressive subsyndromes in childhood is the clinical approach where clinical pictures are drawn, utilizing an inductive method combined with clinical judgement. The second is a statistical approach using multivariate procedures such as exploratory factor analysis. What has not yet emerged is a clear idea of the proportions of prior disorders which shift into major depression, and the proportion of major depressive disorder (MDD) that remain 'uncontaminated' by comorbidity.