Both biological and psychological factors have been implicated in the etiology and treatment of depression. Simple deficit theories in both domains have given way to more complex models that emphasize multiple regulatory systems. Evidence has accumulated that both biological and psychological factors are important, but questions remain as to how they relate to one another and the extent that each contributes to the etiologies of the various subtypes within the disorder. Current research evidence supports the effectiveness of both pharmacological and psychosocial interventions in the treatment of depression. Although the efficacy of the former is more firmly established, the latter (particularly the cognitive-behavioral approaches) may have a preventive capacity that survives the termination of treatment. © 1991 Association for Advancement of Behavior Therapy. All right reserved.