Bipolar Disorder is characterized by mood swings. There are several varieties of the illness. Bipolar Disorder type I, is the classic "Manic Depression" whose extremes are still sometimes misdiagnosed as schizophrenia. Bipolar Disorder type II is similar to type I, except that type IIs don't experience psychosis.

A. Criteria, except for duration, are currently (or most recently) met for a Manic, a Hypomanic, a Mixed, or a Major Depressive Episode.
B. There has previously been at least one Manic Episode or Mixed Episode.
C. The mood symptoms cause clinically significant distress or impairment in social, occupational, or other important areas of functioning.
D. The mood episodes in Criteria A and B are not better accounted for by Schizoaffective Disorder and is not superimposed on Schizophrenia, Schizophreniform Disorder, Delusional Disorder, or Psychotic Disorder Not Otherwise Specified.
E. The mood symptoms in Criteria A and B are not due to the direct physiological effects of a substance (e.g., a drug of abuse, a medication, or other treatment) or a general medical condition (e.g., hyperthyroidism). A. The person currently has, or in the past has had at least one major depressive episode.
B. The person currently has, or in the past has had at least one hypomanic episode:
C.The person has never experienced a manic or mixed episode.
D. Another disorder does not better explain the episode.
E. The symptoms are a cause of great distress or difficulty in functioning at home, work, or other important areas.

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Excerpted from Diagnostic and Statistical Manual of Mental Disorders DSM-IV-TR Fourth Edition