Right-handed healthy volunteers, people with major depression who are currently depressed or have previously been depressed and people with bipolar depression between 18 and 50 years of age may be eligible for this study. Candidates are screened with a medical and psychiatric history, physical examination, electrocardiogram, blood and urine tests. They are also interviewed to evaluate mood, sleep, energy, work and school performance, and social relationships, and asked to answer questions to investigate whether any history of paranoia, panic attacks, obsession, compulsions, suicidal thoughts, eating disturbances, and alcohol or drug abuse is present. They complete rating scales for depression, anxiety, and negative thinking; history of alcohol and tobacco use; physical movement; socioeconomic status; overall level of functioning; and depression type. Finally, they undergo a battery of neuropsychological tests to assess general intelligence, handedness, and specific cognitive abilities, including memory and concentration.