Bipolar II disorder (BP-II) is defined, by DSM-IV, as recurrent episodes of depression and hypomania. Hypomania, according to DSM-IV, requires elevated (euphoric) and/or irritable mood, plus at least three of the following symptoms (four if mood is only irritable): grandiosity, decreased need for sleep, increased talking, racing thoughts, distractibility, overactivity (an increase in goal-directed activity), psychomotor agitation and excessive involvement in risky activities. This observable change in functioning should not be severe enough to cause marked impairment of social or occupational functioning, or to require hospitalisation. The distinction between BP-II and bipolar I disorder (BP-I) is not clearcut. The symptoms of mania (defining BP-I) and hypomania (defining BP-II) are the same, apart from the presence of psychosis in mania, and the distinction is based on the presence of marked impairment associated with mania, i.e. mania is more severe and may require hospitalisation. This is an unclear boundary that can lead to misclassification; however, the fact that hypomania often increases functioning makes the distinction between mania and hypomania clearer. BP-II depression can be syndromal and subsyndromal, and it is the prominent feature of BP-II. It is often a mixed depression, i.e. it has concurrent, usually subsyndromal, hypomanic symptoms. It is the depression that usually leads the patient to seek treatment.DSM-IV bipolar disorders (BP-I, BP-II, cyclothymic disorder and bipolar disorder not otherwise classified, which includes very rapid cycling and recurrent hypomania) are now considered to be part of the 'bipolar spectrum'. This is not included in DSM-IV, but is thought to also include antidepressant/substance-associated hypomania, cyclothymic temperament (a trait of highly unstable mood, thinking and behaviour), unipolar mixed depression and highly recurrent unipolar depression.BP-II is underdiagnosed in clinical practice, and its pharmacological treatment is understudied. Underdiagnosis is demonstrated by recent epidemiological studies. While, in DSM-IV, BP-II is reported to have a lifetime community prevalence of 0.5%, epidemiological studies have instead found that it has a lifetime community prevalence (including the bipolar spectrum) of around 5%. In depressed outpatients, one in two may have BP-II. The recent increased diagnosing of BP-II in research settings is related to several factors, including the introduction of the use of semi-structured interviews by trained research clinicians, a relaxation of diagnostic criteria such that the minimum duration of hypomania is now less than the 4 days stipulated by DSM-IV, and a probing for a history of hypomania focused more on overactivity (increased goal-directed activity) than on mood change (although this is still required for a diagnosis of hypomania). Guidelines on the treatment of BP-II are mainly consensus based and tend to follow those for the treatment of BP-I, because there have been few controlled studies of the treatment of BP-II. The current, limited evidence supports the following lines of treatment for BP-II. Hypomania is likely to respond to the same agents useful for mania, i.e. mood-stabilising agents such as lithium and valproate, and the second-generation antipsychotics (i.e. olanzapine, quetiapine, risperidone, ziprasidone, aripiprazole). Hypomania should be treated even if associated with overfunctioning, because a depression often soon follows hypomania (the hypomania-depression cycle). For the treatment of acute BP-II depression, two controlled studies of quetiapine have not found clearcut positive effects. Naturalistic studies, although open to several biases, have found antidepressants in acute BP-II depression to be as effective as in unipolar depression; however, one recent large controlled study (mainly in patients with BP-I) has found antidepressants to be no more effective than placebo. Results from naturalistic studies and clinical observations on mixed depression, while in need of replication in controlled studies, indicate that antidepressants may worsen the concurrent intradepression hypomanic symptoms. The only preventive treatment for both depression and hypomania that is supported by several, albeit older, controlled studies is lithium. Lamotrigine has shown some efficacy in delaying depression recurrences, but there have also been several negative unpublished studies of the drug in this indication.