The most common cause of dementia occurring in mid- to late-life is Alzheimer's disease (AD). Some cases of AD, particularly those of early onset, are familial and inherited as autosomal dominant disorders linked to the presence of mutant genes that encode the amyloid precursor protein (APP) or the presenilins (PS1 or PS2). These mutant gene products cause dysfunction/death of vulnerable populations of nerve cells important in memory, higher cognitive processes, and behavior. AD affects 7-10% of individuals > 65 years of age and perhaps 40% of individuals > 80 years of age. For the late-onset cases, the principal risk factors are age and apolipoprotein (apoE) allele type, with apoE4 allele being a susceptibility factor. In this review, we briefly discuss the clinical syndrome of AD and the neurobiology/neuropathology of the disease and then focus attention on mutant genes linked to autosomal dominant familial AD (FAD), the biology of the proteins encoded by these genes, and the recent exciting progress in investigations of genetically engineered animal models that express these mutant genes and develop some features of AD.