Dementia has traditionally been viewed as a global, undifferentiated impairment of intellect and memory. The implication is that patients with dementia share a common clinical syndrome. It is, however, now recognized that different dementing diseases give rise to distinct patterns of mental change, reflecting differences in the topographical distribution of pathological change within the brain. Contrary to the traditional view, analysis of the characteristics of dementia can contribute substantially to differential diagnosis. Indeed, since many patients with a dementing illness exhibit few physical signs, evaluation of the mental changes may be critical to diagnostic accuracy. With the advent of new therapies for dementia, precise diagnosis has become increasingly important. Moreover, understanding of patients’ symptom pattern provides a rational basis for patient management and for advice to carers.