This paper reviews some of the essential facts about depression in adults. The term can refer to a symptom, a syndrome or a diagnosis. As a symptom, it occurs in many mental disorders and physical diseases. The boundaries of depression are reviewed: depression is biologically distinct from bipolar affective disorder, but it overlaps with the other common neurotic conditions; the spectrum of depressive disorders, melancholia, depressive adjustment reactions and dysthymia is described. Categorical classifications of depression have many problems, but are currently necessary. The epidemiology of depression in adulthood is presented with reference to old age, after childbirth, its prognosis and long-term disability and the limitations of short-term treatments. The immediate causes of depression include complex interactions between life events and their personal meaning influenced by personality factors and neuroticism. Ethological perspectives indicate that depression may have both adaptive and maladaptive aspects.