The diagnosis of bipolar disorder is made according to the fulfillment of symptom criteria as operationally defined by the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition. These criteria encompass both cross-sectional and historical data, along with pertinent corroborative features such as family history, chronobiological sensitivities, treatment outcomes, longitudinal course, and patterns of recurrence. In addition, one classifies a current illness state as involving pure mania, hypomania, a mixed episode, a depressed phase, or euthymia. Mixed states, as originally described by Kraepelin, were thought to represent transitional phenomena en route to the emergence of a frank manic or depressive episode. Today, however, mixed episodes are thought to comprise an important subgroup of syndromically ill individuals with bipolar disorder.