This document defines bipolar disorder and discusses its prevalence and course. It states that bipolar disorder is characterized by alternating periods of mania/hypomania and depression. The DSM-IV categorizes it into four subtypes: bipolar I, bipolar II, cyclothymia, and bipolar NOS. Bipolar I is defined by one or more manic or mixed episodes, while bipolar II involves one or more hypomanic episodes in addition to major depressive episodes but no full mania. Lifetime prevalence rates for bipolar I and II are around 1% each. The course of bipolar disorder typically involves recurrent mood episodes throughout one's lifetime.