This chapter describes the spectrum of mood disorders associated with attention-deficit hyperactivity disorder (ADHD). It explains the incidence of overlap between these two diagnostic categories, and the implications of such overlap for research and clinical practice. Research on the role of emotion in cognitive functioning has highlighted the fact that much of the influence of emotion on cognition is instantaneous and outside the realm of conscious experience. The chapter discusses the incidence of mood disorders among adults with ADHD. Although many types of mood disorders are described in DSM-IV-TR, the mood disorders reported in studies of comorbidity generally include the following types: major depressive disorder (MDD), dysthymic disorder, and bipolar disorder (BPD). When both ADHD and a mood disorder are present in an adult patient, it is important to assess the relative severity of each and to plan carefully to prioritize and provide optimal treatment for each of the disorders.