Bipolar disorder (manic-depressive illness) is a mood disorder, which means that the symptoms are disturbances or abnormalities of mood. Major depression is a more common illness, the symptoms of which are mainly those of 'low' mood.

Bipolar disorder involves episodes of both serious mania and depression. The person's mood swings from excessively 'high' and irritable, to sad and hopeless, and then back again, with periods of normal mood in between.

Different from normal mood states of happiness and sadness, symptoms of manic-depressive illness can be severe and life threatening. However, because many artists, musicians and writers have suffered from bipolar illness, the effect of the illness has sometimes been trivialised, and regarded in some way as beneficial for artistic creativity. In fact, for those afflicted with the illness, it is extremely distressing and disruptive.

Bipolar disorder is the third most common mood disorder after major depression and dysthymic disorder. It affects about 1% of adults during their lifetime. Symptoms typically begin during adolescence or early adulthood, and continue to recur throughout life. Men and women are equally likely to develop this disabling illness.