This chapter discusses the prevalence of depressive disorders in children and adolescents, unipolar depression, puberty and depression, comorbidity, and environmental risk factors for depression. Only a small minority of disturbed children are referred for psychiatric treatment. Thus estimates of the rates of depression in children and adolescents cannot be determined from clinical data. Much less attention has been paid to bipolar disorders (BPDs) in the child and adolescent epidemiological literature. Studies of adults have documented that women have 1.5-3 times more current and lifetime unipolar depression than do men. The National Institute of Mental Health study of puberty and psychopathology found negative associations between testosterone: oestradiol ratio, sex hormone binding globulin, and androstenedione concentration and negative emotional tone in boys. Recent review shows that comorbidity between depression, conduct disorder, anxiety, attention deficit hyperactivity disorder (ADHD) and substance abuse is common in the general population of children and adolescents.