Bipolar mood disorder is a kind of mood disorder in which the person experiences episodes of mania as well as episodes of depression. The onset period for developing the disorder is usually late twenties to early thirties. The first episode may be either manic or depressive. Bipolar individuals tend to have more episodes than unipolar patients, and the disorder tends to follow a long-term, episodic course, with mixed prognosis. Manic episodes usually remit within one to three months without treatment, while depressive episodes typically last three times longer. The lifetime prevalence of bipolar mood disorders is about 1 percent of the population. No gender differences have been found regarding the risk for developing the disorder.

There are three types of bipolar mood disorders: bipolar I; bipolar II; and cyclothymia. To meet the diagnostic criteria, the mood disorders must be severe and long enough to cause social and occupational dysfunction. All three types involve either manic or hypomanic episodes. Individuals with bipolar I are characterized by having experienced at least one manic episode, and although they do not need to have experienced a depressive episode to meet diagnostic criteria, most of them actually have episodes of major depression in addition to manic episodes. Manic episodes are distinct periods in which mood is elevated, expansive, or irritable for at least a week. If the symptoms are elevated or expansive, the individual only needs to meet three of the following criteria: inflated self-esteem or grandiosity; decreased need for sleep, yet no feelings of fatigue; more talkative than usual or feeling pressure to keep talking; flight of ideas or subjective experience of thoughts racing; distractibility; increase in goal directed activity or psychomotor agitation; and excessive involvement in pleasurable activities that have a high potential for painful experiences. If the individual expresses irritable symptoms instead, they must meet four of the above seven criteria. Manic episodes trump everything else--meaning that the presence of a manic episode immediately identifies the disorder as bipolar, and manic episodes usually lead to severe functional impairment.

Individuals in the bipolar II category have experienced at least one depressive episode, but do not have full-blown manic episodes. Instead, they have periods of hypomania: periods of increased energy that are less severe and shorter than manic episodes. Hypomanic episodes, which only last for about four days and do not include psychotic symptoms, produce noticeable impairment, but not enough to lead to social or occupational dysfunction. Cyclothymia is a chronic but less severe form of mood disorder, or the bipolar equivalent of dysthymia. Individuals who fit the criteria for cyclothymia usually have rapid cycling of hypomanic and depressive episodes and do not have a history of manic or major depressive episodes.