Bipolar disorder is a highly recurrent and chronic psychiatric condition that shortens life expectancy, causes functional impairment and disruption to social, work and family life. Several forms of bipolar disorder are recognised, including both bipolar I and bipolar II disorder. Bipolar I is characterised by recurrent episodes of depression and mania whereas bipolar II disorder is characterised by recurrent depression and hypomania, a milder form of mania. There has been debate concerning the definition of hypomania since at least the 1970s. The main areas of argument focus on the minimum duration of hypomania, its stem criteria and the number of symptoms required for diagnosis. Arriving at the correct definition of hypomania is a key diagnostic issue. There is increasing evidence for the existence of a broad spectrum of bipolar disorders, and data demonstrating the clinical validity of modifying some of the criteria for hypomania are reviewed here.