Alzheimer's disease, the most common cause of dementia in the elderly, is rapidly becoming epidemic in the western world, with major social and economic ramifications. Thus enormous international scientific efforts are being made to increase our understanding of the pathogenesis of this disease, with the eventual goal of developing beneficial therapy. The two major neuropathological hallmarks of Alzheimer's disease (AD) are extracellular senile plaques, the principal component of which is the A beta amyloid peptide, and intraneuronal neurofibrillary tangles, which are composed of aggregated tau protein in the form of paired helical filaments (PHF). In the past decade, since the major proteinaceous components of these pathological markers have been identified, great strides have been made in elucidating the biochemical processes which may underlie their abnormal deposition and aggregation in Alzheimer's disease. Simultaneously, extensive population genetic analyses have identified mutations in the A beta amyloid precursor protein (APP) in a small number of pedigrees with familial Alzheimer's disease (FAD) whilst other FAD cases have been linked to an, as yet, unidentified marker on chromosome 14. Most recently, inheritance of the type 4 allele of apolipoprotein E has also been identified as a risk factor in sporadic AD. The challenge facing scientists now is to incorporate this wealth of exciting new biochemical and genetic data into a coherent model which can explain the long established neurochemical and histopathological lesions characteristic of AD.