Depressive disorders are a collection of mental illnesses that emerge with their highest incident risk rate in the second decade of life. These conditions are aetiologically and clinically heterogeneous. Longitudinal studies of depressed adolescents from cohort and clinical populations demonstrate marked individual differences in outcome into the third decade of life. Randomized clinical trials show that children, adolescent and adults may not be equally sensitive to psychological or pharmacological treatments at least in the weeks following end of treatment.