The historical events in the evolution of Alzheimer's disease are reviewed, including the initial description by Alois Alzheimer and the subsequent controversy regarding the nosological specificity of this entity. The similarity of senile dementia and Alzheimer's disease is emphasized. The basis for the modern concept of Alzheimer's disease as premature or accelerated aging is included in the review. The pathological correlates of the major categories of adult dementia have been described. The traditional criteria of neurofibrillary tangles and senile plaques have been re-evaluated using the current insight into these changes afforded by electron microscopy and biochemistry. The significance of amyloid has been described because it occurs within the senile plaque and also as the essential component of congophilic angiopathy. The new information regarding neuronal cell counts and the loss of choline acetyltransferase has been evaluated in terms of an indication of a pathogenic mechanism of Alzheimer's disease. The current understanding of normal pressure hydrocephalus, Creutzfeldt-Jakob disease, and multi-infarct dementia has been described. Brain biopsy in dementia has been described as having diagnostic, research, pathogenic, and prognostic value. The precautions involving the performance and handling of the biopsy have been stressed, particularly because these procedures involve conditions of possible slow virus etiology. The polemic for Alzheimer's disease as aging or slow virus infection has been summarized. At this time a consideration seems justified that Alzheimer's disease is an age-related, slow virus disease due to a hitherto unknown immune defect. Aging as an etiological agent must be clarified before Alzheimer's disease, in any form, can be considered to be an inevitable consequence of longevity.