While the idea that individuals with developmental disabilities (DD) can suffer from mental illness has been largely accepted since the late 1980's, this is still an underserved and poorly studied population. In particular, mood disorders have traditionally been misdiagnosed, under-recognized and poorly treated. Through the years, reported rates of mood disorders in adults with DD have varied widely. Recent epidemiological studies have focused on community samples and find rates of mood disorders from 3 to 8.1%. Mood disorders are found to be more prevalent than psychotic disorders or anxiety disorders. The empirical literature on assessment of mood symptoms in adults with DD is limited, particularly in individuals with severe and profound DD. Several tools have been developed to assist in identification and diagnosis. However, the work on their psychometric properties and validity studies is still quite limited. The treatment literature is sparse and focused primarily on pharmacotherapy. This review considers the epidemiology and diagnosis of mood disorders in individuals with DD. Recent developments in assessment are reviewed. The literature regarding pharmacological treatment with antidepressants, mood stabilizers, electroconvulsive therapy and antipsychotics is summarized and the current state of psychological treatments for mood disorders in persons with DD is presented. Implications for clinical care and future research are considered.