Chapter 6 addresses major depressive disorder and bipolar disorder. Information concerning prevalence, demographic, multicultural, and developmental findings pertaining to each disorder is reviewed from an empirical perspective. Etiologic theories are discussed, including a review of genetic, neurotransmitter, molecular, structural, and functional neuroimaging findings. Pharmacological interventions are reviewed, and specific medications are discussed. Research concerning the efficacy of antidepressants in the treatment of MDD and findings regarding suicidal behavior are discussed. Alternative interventions in the treatment of both disorders are briefly reviewed.