Persons with intellectual disabilities (ID) are at higher risk of developing a depressive disorder than are members of the general population. This article attempts to answer the question of how best to adapt psychological assessment and intervention for depressive disorder to the needs of adults with ID. Current knowledge suggests that a diagnostic assessment conducted by a multidisciplinary team is the best way to evaluate the influence of biological and psychosocial factors that may contribute to the development of depressive symptoms, and identify potential differential diagnoses. The greater the person's communication and cognitive deficits are, the greater the clinician's difficulties are in assessing and treating depressive disorder in persons with ID. Family and caregivers of such persons are therefore essential partners in assessment and therapeutic process. The recommended interventions are education, skill development, environment modification, and cognitive and behavioural therapy. All of these must, however, be adapted to the person's level of functioning. While the use of antidepressants may prove necessary, side effects must be carefully monitored.