Hypersomnia is prevalent and persistent across mood disorders. This review has two aims: (1) to synthesize the research that has accrued on hypersomnia in mood disorders and (2) to identify an agenda for future research that advances knowledge on this critical, but understudied, feature of mood disorders. We begin by reviewing the state-of-the-science on the diagnosis, epidemiology and course of hypersomnia in Major Depressive Disorder, Bipolar Disorder, Dysthymic Disorder, and Seasonal Affective Disorder. We then address key measurement and assessment issues, particularly those arising from the use of objective methods. This section identifies a need to explore whether hypersomnia is a disorder of Time in Bed (TIB) rather than a disorder of Total Sleep Time (TST), or whether presentations of TIB vs. TST represent two subtypes of hypersomnia with differing etiologies. Established and proposed mechanisms contributing to hypersomnia are then highlighted, including the possibility that hypersomnia is an important mechanism contributing to the maintenance of mood disorder symptoms. We then move on to review the small body of literature on pharmacological interventions for hypersomnia in mood disorders. Though non-pharmacological treatments targeting hypersomnia have not yet been developed, we offer initial guidelines for such treatments and conclude with an agenda for future research.