Depressive disorders--both major depression and other less severe but nonetheless clinically significant depressions--are common comorbidities, components, or complications of dementia. Depression with reversible cognitive impairment may be a prodrome for dementia rather than a separate and distinct disorder. Recent research has demonstrated that both the diagnosis of major depression and the assessment of typical depressive symptoms can be conducted reliably, even in patients with mild-to-moderate levels of cognitive impairment. Self-ratings of depressive symptoms with the Geriatric Depression Scale remain valid in patients with Mini-Mental State Examination scores of at least 15. Among interviewer-administered instruments, the Hamilton Rating Scale for Depression and the Cornell Scale are the best established. Potential difficulties with assessment include problems with ascertainment (because families, in general, report greater depression in patients than do clinicians) and the ambiguity of symptoms (because apathy and related symptoms can result from both depression and Alzheimer's disease). Brain changes due to Alzheimer's disease may lead to fundamental differences in drug responses. Nevertheless, randomized clinical trials have demonstrated that depression in dementia responds to specific psychopharmacologic or psychosocial treatments.