Cyclothymic disorder is a subtype of bipolar disorder included in the Diagnostic and Statistical Manual of Mental Disorders since 1980, but largely neglected in research. Additionally, it is rarely diagnosed clinically, in spite of evidence that it may be the most prevalent form of bipolar disorder. Neglect has contributed to confusion about the diagnosis and clinical presentation of cyclothymic disorder. Its status as a mood disorder is also ambiguous due to overlap in terminology and symptoms with temperament and personality disorders. Subthreshold bipolar disorder appears more prevalent among young people than previously thought, and follows a range of trajectories from remission to escalation-raising questions about risk factors and traits associated with the varied course. Cyclothymic disorder may be an important diathesis for major mood disorders. Constructs such as cyclothymic disorder link major mood disorder and peri-clinical fluctuations of mood, thus warranting a prominent role in dimensional models of mood and psychopathology. Current evidence indicates that cyclothymic disorder is a prevalent and highly impairing disorder on the bipolar spectrum, with the potential to make unique contributions to our understanding of the risk factors and outcomes associated with bipolar disorder. The inclusion of cyclothymic disorder in future research studies is essential to accurate diagnosis and effective treatment for the full spectrum of bipolar disorder, as well as understanding the developmental trajectory of bipolar spectrum disorders.