1. Major depressive disorder is recognized by different, frequently recurring patterns of signs and symptoms that are suggestive of specific subtypes of the disorder, including melancholic, atypical, and psychotic. 2. MDD occurs at younger ages, twice as frequently in women, in conjunction with other psychiatric disorders, in persons with a family history of mood disorders, and is associated with high use of medical services. 3. Research on the biological basis of depression has focused on neurotransmission, neuroendocrine dysregulation, and genetic transmission. 4. Advances in understanding MDD have important implications for nursing assessment, diagnosis, outcome planning, intervention, and evaluation with depressed clients.