The authors review the literature on the clinical and economic impact of unrecognized and inadequately treated bipolar disorder, highlighting the need to improve identification and treatment of this disabling disorder. Epidemiologic data on prevalence, diagnosis, and treatment of bipolar disorder (including subthreshold conditions) are presented, including data from the recent National Comorbidity Survey Replication. Clinical factors that contribute to misdiagnosis and resulting inappropriate treatment of bipolar disorder are reviewed as well as negative clinical consequences of such misdiagnosis and inappropriate treatment. The economic impact of underrecognized and inadequately treated bipolar disorder is discussed. The data provide empirical support for screening all patients diagnosed with depression for evidence of bipolar disorder before initiating treatment, to ensure that bipolar illness is not misdiagnosed and treated as unipolar mood disorder. Readers are referred to performance measures and treatment resources assembled by the STAndards for BipoLar Excellence (STABLE) Project to help clinicians screen more accurately for bipolar disorder.