The study of Alzheimer's disease (AD) has had more than its share of controversy. In the last few years, however, considerable consensus has been developed that certain molecular events in the brain occur many years or decades before clinical symptoms, and a rough outline of the pathogenic cascade has emerged from the studies of many investigators. Progress in two broad areas has led to this consensus: molecular genetics and biochemical pathology. In this article, the exciting new understanding of the pathogenesis of AD is reviewed and its affect on the patient population is discussed.