More than 50% of patients with bipolar disorder fulfil criteria for an additional (lifetime) diagnosis of a psychiatric disorder. Furthermore, bipolar patients are at a high risk to suffer from a medical disorder. Anxiety disorders, substance abuse disorders, personality disorders and attention deficit hyperactivity disorders occur at high frequency. Such "comorbid"disorders complicate course and outcome of bipolar disorder. Subjects with a bipolar disorder plus a comorbid psychiatric disorder tend to have lower functioning, an earlier age of onset and less favourable treatment response. Evidence concerning treatment strategies for bipolar disorder plus a comorbid psychiatric disorder is sparse. There is disagreement, whether the co-occurrence of more than one diagnosis in bipolar disorder justifies the idea of separate entities or rather reflects a broad spectrum.