This article reviews the prevalence and incidence of mental disorders in older adults. The authors outline the epidemiologic challenges in determining the frequency of mental disorders in late-life and discuss issues that are critical for understanding the prevalence of the disorders and for reviewing the evidence from epidemiologic studies of mental disorders in this population. The authors summarize the epidemiologic data for depression, anxiety, dementia, schizophrenia, and alcoholism. Also included is a discussion of risk factors and outcomes of these disorders and a discussion of the implications of these epidemiologic findings for geriatric medicine.