Unipolar depression is a relatively common psychiatric condition in young people. Clinical signs and symptoms at presentation can vary with age, with children showing different characteristics to adolescents. Depressive episodes before 18 years of age are associated with increased risk of recurrence into adulthood, with some early-onset cases showing a chronic course into adult life. Both recurrent and chronic cases in young adulthood are associated with long-term suffering and impairment. Depressive episodes invariably present with a range of other non-depressive disorders, such that co-morbid presentation is the rule rather than the exception. These early-onset depressions are heterogeneous in their origins and outcomes. Treatment of a depressive episode results in full remission in about two-thirds of cases, but we know little of the effects of treatment on subsequent relapse and recurrence. A better understanding of the causes and the pathophysiology of these disorders would greatly inform treatment decision-making and public health policy.