The authors provide an overview of the diagnosis, course, and treatment of bipolar II disorder, a distinct subtype that is often misdiagnosed as unipolar depression or bipolar I disorder. They discuss research suggesting that underdiagnosis of bipolar II disorder reflects a failure to identify subthreshold expression of mania (hypomania). The course of bipolar II disorder is different from that of bipolar I disorder or unipolar depression, with distinct differences in rates of recovery, clinical features, and number of episodes. The risk of suicide appears to be particularly elevated. High rates of comorbid disorders have been reported, including substance abuse or dependence, anxiety disorders, and personality disorders. Few definitive studies exist on which to base conclusions about the differential efficacy of various treatment strategies in bipolar II disorder and bipolar I disorder. Preliminary studies suggest that the newer anticonvulsants may be of benefit for patients with bipolar II disorder, while other data suggest that there may be a greater role for antidepressant medications.