The nosology of dementia has evolved from the simple distinctions between senility, degenerative dementia, and vascular insults that existed a few decades ago. The differentiation of normal cognitive aging from very mild dementia has sharpened considerably. There currently is great interest in identifying the earliest symptomatic stages of AD, particularly in relation to the availability of therapeutic agents. Evidence is accumulating to suggest that minimal cognitive decline in the elderly generally signifies AD when no other cause is readily apparent. On the other hand, appreciation of the heterogeneity of AD and its clinical and pathologic overlap with other dementing disorders has blurred previous classification systems. Rather than viewing AD and vascular dementia as dichotomous entities, for example, it may be more relevant to consider their additive or synergistic interactions in producing dementia. Further elucidation of the nature and contribution of genetic factors in AD and related disorders will accelerate the use of genotypephenotype correlations in dementia classifications.