Bipolar disorder is both prevalent and disabling. Surveys suggest that patients with bipolar disorder are often misdiagnosed on initial presentation, most often with major depressive disorder. These patients may receive ineffective treatment, which, in some cases, actually worsens outcome, either by inducing manic or mixed states or by increasing mood cycling. Major contributors to misdiagnosis include incomplete history and lack of patient insight as well as presence of psychiatric comorbidity, such as anxiety or substance use disorders. Careful screening for current and past symptoms of mania or hypomania, as well as close clinical follow-up, can help to reduce misdiagnosis.