Alzheimer's disease (AD), the most common dementing disorder of late life, is a major cause of disability and death in the elderly. Neurobiological, genetic, and molecular studies have defined the vulnerable neural systems, abnormalities in cytoskeletal proteins in neurons, the biology of the beta-amyloid precursor protein (APP) and beta-amyloid (A beta, beta A4), and several APP mutations linked to the disease. More recently, investigators have begun to develop animal models essential for delineating pathogenetic mechanisms and for developing and testing new therapies for treating AD in humans. This review focuses primarily on recent progress in investigation of animal models of AD (including aged nonhuman primates and transgenic mice), which have begun to clarify some of the questions raised by investigation of the disease in humans.