First Published in 2002. In common usage, the term "depression" can refer to the state of being sad or blue, but it also signifies a serious clinical syndrome that affects approximately 10 percent of people at some point in their lives. This clinical syndrome may occur as a primary illness or as a complication of ("secondary to") another mental disorder such as schizophrenia, a medical condition such as hypothyroidism, or the effects of a drug. Based on studies of clinical courses and outcomes, treatment responses, and familial patterns of depression, primary depressive illness is dichotomized into unipolar (depressions only) and bipolar. In bipolar disorder, or manic-depressive illness, depressions are interspersed with manias- periods of elevated mood, high energy, and lack of sleep. Bipolar disorder is described in a separate volume.