Epidemiological and clinical studies have reported a high prevalence of anxiety symptoms in bipolar disorder, either in manic or depressive episodes, although these symptoms do not always meet criteria for a specific anxiety disorder. In addition to anxiety symptoms, bipolar disorder frequently presents with co-morbid axis I conditions, with anxiety disorders being the most common co-morbidity. Therefore, the objective of this article is to review clinical and epidemiological studies that have investigated the association between bipolar disorder and anxiety. Available data on the efficacy of treatments for bipolar disorder and co-morbid anxiety disorders are also reviewed. Existing guidelines do recognize that co-morbid anxiety has a negative impact on the course and outcome of bipolar disorder; however, there have been very few double-blind, controlled trials examining the treatment response of patients with bipolar disorder and co-occurring anxiety disorders. There is some positive evidence for quetiapine, olanzapine in combination with fluoxetine or lithium, and lamotrigine with lithium, and negative evidence for risperidone. Other therapies used for bipolar disorder, including several mood stabilizers, antidepressants, atypical antipsychotics and benzodiazepines, have been shown to reduce anxiety symptoms, but specific data for their effects in patients with anxiety symptoms co-morbid with bipolar disorder are not available. The co-occurrence of anxiety and bipolar disorder has implications for diagnosis, clinical outcome, treatment and prognosis. Careful screening for co-morbid anxiety symptoms and disorders is warranted when diagnosing and treating patients with bipolar disorder.