The objective of this book is to review the impact of genetic variation on risk of human disease at the different major levels of organization: cells, individuals, families, and populations. The volume begins with a discussion of sources and rates of mutation which ultimately give rise to the vast amount of extant genetic variation. This is followed by presentations of current understanding of how genetic variation is maintained within and among populations. The volume ends with discussions of the implications of such variation for understanding the evolution of our species. This collection gives an unusually broad treatment of the subject, with chapters from some of the leading workers in the field. James Neel's chapter on human consanguinity effects and M. Otake's on the genetic effects of radiation associated with the dropping of the Hiroshima and Nagasaki atomic bombs should be singled out for special emphasis. As an up-to-date overview of ongoing research, this work will be of interest to a wide range of workers in the fields of human population genetics, evolution, and epidemiology.