Depression, as a heterogeneous collection of disorders, is likely to include subgroups that are more genetic in origin. In common with other neuropsychiatric disorders such as schizophrenia, Alzheimer's disease and Huntington's disease, earlier age at onset in depression is associated with higher genetic loading and poorer long-term outcome. Adolescents and young adults with depression are also at high risk of developing a bipolar illness. This article reviews depressive illnesses that occur for the first time in adolescence and young adulthood. Case studies are used to discuss atypical presentations and the evolving concept of bipolar-spectrum disorders.