Alzheimer's disease (AD) is a neurodegenerative disorder that is the most common cause of dementia in the elderly. It is a clinical-pathologic entity characterized by progressive dementia associated with the neuropathologic hallmarks of Abeta amyloid plaques, neurofibrillary tangles (NFTs), neuronal loss, and amyloid angiopathy. Three "causative" AD genes (i.e., genes in which a mutation is sufficient to result in clinical AD) for early-onset familial Alzheimer's disease (FAD) and one "susceptibility" gene that affects risk and age of onset of AD in familial and sporadic late-onset AD have been identified. The three causative genes are the amyloid precursor protein (APP gene) on chromosome 21, the presenilin-1 gene on chromosome 14, and the presenilin-2 gene on chromosome 1. The susceptibility gene is the apolipoprotein E (APOE) gene on chromosome 19. Investigations of the normal and aberrant function of these genes will provide insights into the mechanisms underlying AD and will suggest new strategies for therapeutic intervention.