In this article, the authors review the genetic investigations in Alzheimer's disease employing epidemiologic study techniques--mainly case-control, twin, and cumulative risk studies--as well as pedigree and genetic linkage studies and discuss the implications these studies have for identifying and characterizing a possible genetic etiology of Alzheimer's disease. Some of the methodologic issues associated with each of these techniques and the more general problems associated with identifying genetic factors in Alzheimer's disease are discussed, and some directions and prospects for future research in this area are indicated.