Bipolar disorder, formerly called manic-depressive illness, is one of several disorders known as mood disorders. Mania and depression alone or in combination are the hallmarks of the mood disorders. Mania is characterized by a feeling of euphoria in which the individual has grandiose ideas, exhibits boundless energy, needs little sleep, and exhibits great self-assurance. While in a manic depressive disorder state people's thoughts race, they speak too fast, and they demonstrate poor judgment. Manics may impulsively spend too much money, commit sexual indiscretions, and alienate people with their irritability and impatience. Hypomania refers to a milder form of mania that is an excessive amount of elation, but does not significantly impair the individual's life.

Depression can be characterized by many symptoms, including feelings of worthlessness, guilt, and sadness. When one is depressed, life seems empty and overwhelming. The depressed individual has difficulty concentrating, cannot make decisions, lacks confidence, and cannot enjoy activities that previously were pleasurable. Physical symptoms may include gaining or losing weight, sleeping too much or too little, agitation, or lethargy. Depressed individuals may be preoccupied with death or suicide. They may believe that they have committed the unpardonable sin and that loved ones would be better off without them.

Bipolar disorder is so named because those afflicted with it experience both mania and depression, in contrast to those with unipolar disorders, who experience only one extreme, usually depression. Bipolar disorders are categorized into two types, Bipolar I and Bipolar II. In Bipolar I the individual experiences both mania and depression; in Bipolar II the individual experiences hypomania and depression. Mania or hypomania is the key to diagnosing bipolar disorder. A person who experiences a manic state, even once is presumed to have bipolar disorder. Manic and depressive states may immediately precede or follow one another or may be separated by long time intervals, and the individual may have more episodes of one pole than the other. Some individuals, known as rapid cycles, will experience four or more episodes per year.

The age of onset for bipolar disorder is younger than for unipolar depression and usually begins in the late teens or twenties but seldom begins after age 40. In some cases, it is preceded by a disorder named cyclothymic, which is a milder form of mood disorder, characterized by marked moodiness and mood swings for at least two years. Bipolar disorder is a chronic disorder and even with treatment less than half of the individuals who experience it have gone five years without a manic or a depressive episode. People with bipolar are at risk for suicide in the depressive phase and are more prone to accidental death in the manic phase due to impulsiveness and poor judgment.

The causes of bipolar disorder are unclear, but it is probably determined by multiple factors. Family and adoption studies have consistently indicated a genetic predisposition toward mood disorders. First-degree relatives of persons with bipolar disorder are much more likely than the general population to experience bipolar depression, depression treatment, unipolar depression, and anxiety. At this point, however, there is no clear evidence that a particular gene is linked to the transmission of bipolar disorder; instead, it seems that family history increases vulnerability to several disorders.

Stressful life events may precipitate episodes of mania or depression, but do not seem to be the primary cause of the bipolar disorder. Psychosocial factors such as attribution style, learned helplessness, attitudes, and interpersonal relationships all seem to be correlated with bipolar disorder but have not been identified as causes; they are often the result of having such a disorder. It seems that a genetic vulnerability coupled with stressful psychological and sociocultural events may result in bipolar disorder.