Depression is the most usual mental disorder in the elderly, but underdiagnosed and undertreated. Its prevalence is variable and depends on type and severity of episode. Nevertheless, even subsyndromic depression needs to be correctly treated. Depressive symptomatology observed in the elderly is often similar to adult presentation, but it can be masked and difficult to recognise. The different clinical features are described with underlining their particularities. Secondary depressions are also evoked with individualisation of "vascular" depression and its etiopathogenic hypotheses in relationship with observations given by cerebral neuro-imagery. Risk factors of depression in old age are known, but recent studies have reviewed some of them, particularly by distinguishing late onset depression and early onset depression. According to therapeutic response and prognosis, it appears necessary to better discriminate them. Risk of dementia after depression seems to be related with type of depressive episode and with the treatment efficacy. Finally, the problem of detection of depression in old age is discussed with a suggestion to use assessment instruments as the mini-GDS in all medical practices, to optimise diagnosis and management.