This paper reviews research on the relationship between age and depression in adulthood, with a focus on depression in late life. Age differences in prevalence rates of major depression and depressive symptomatology raise questions about presentation and measurement of depression across adulthood, and suggest a changing salience of risk factors for depression from young adulthood through old age as well as to cohort differences in risk for depression. Applying a developmental perspective on biological change, psychological adaptation, and stress processes throughout adulthood shows that risk for depression onset in young adults is typified more through psychological vulnerability and stress, as well as genetic factors, while risk for depression in older adults typified more through comorbid medical and neurological disorder. Implications for research and clinical practice are discussed. This review of the relationship of age to depression shows that the study of psychopathology and adult development can inform each other.