Dementia in the adult population is a major and growing medical and social problem. It occurs at all ages, but increases with advancing age, so that the largest group of demented patients is in the older age groups. Dementia has its highest rate in the population over age 75 years, the age group that is increasing at a higher rate than any other. Recent scientific advances give promise of major increases in knowledge and skills that will result in better diagnostic tests and methods of treatment. Dementia, because it interferes with the dignity and independence of the person, causes widespread suffering, not only for the person affected but also for families, friends, and caretakers. The expense of long-term care, either at home or in a nursing facility, can be overwhelming—it has been estimated at $40 billion a year for people aged 65 years and older. Proper identification of the disease state responsible for dementia in the individual patient is critical to management. Some diseases that produce dementia can be arrested or reversed (eg, benign mass lesions of the brain, intoxication, infections, and metabolic and nutritional disorders). Some dementing diseases are infectious, and the tissues and bodily fluids require special handling to prevent transmission. Some dementing diseases are inherited, and this has important social and ethical consequences. The value of therapeutic trials for dementia that are now in progress, and of the epidemiologic studies of prevalence and incidence that are under way in delineated populations, is limited by the criteria and accuracy of diagnosis. It is important to distinguish the early stages of dementia, the clinical behavioral state, from the nonprogressive cognitive changes known to occur in otherwise normal aging. This is not always possible. Serial examination of the patient over time is still the best way to do this. Neuropsychological tests have been developed, and efforts continue to refine them so that dementia can be identified and distinguished from other mental states that may be confused with it. It is important to identify the specific pathological state that produces dementia in the individual patient. The usual clinical methods for doing this are sufficient for certain conditions. Brain-imaging methods and development of biochemical and genetic markers have advanced and show a stunning promise for the future. At present, diagnoses of many of the important dementing diseases can be confirmed or denied with certainty only at autopsy. Pathological diagnosis is the standard of accuracy of clinical diagnosis. Few of the dementing diseases can be studied in an animal model. Pathological studies, while monitoring clinical diagnosis, are providing new knowledge that can be expected to lead to effective antemortem diagnosis of dementing diseases that are now undiagnosable while the patient is alive. In an effort to assess the current state of knowledge about the differential diagnosis of the dementing diseases, the National Institute on Aging, the National Institute of Neurological and Communicative Disorders and Stroke, and the National Institute of Mental Health in conjunction with the National Institutes of Health's Office of Medical Applications of Research convened this conference. Following 11/2 days of presentations by experts in the relevant fields and presentations by public organizations involved with the dementias, a consensus panel consisting of representatives from neurology, psychiatry, geriatric medicine, epidemiology, psychology, family practice, neuropathology, nursing, and the public considered the evidence and formulated a consensus statement responding to these questions: What is dementia? What are the dementing diseases, and which of them can be readily arrested or reversed? What should be included in the initial evaluation of dementia? What diagnostic tests should be performed, and when are these tests indicated? What are the priorities for future research on diagnosing the dementias?