A look at how the concept of "risk factor" has influenced public health and preventive medicine, with an emphasis upon the study of heart disease. The greatest revolutions in twentieth-century public health and preventive medicine have been the concepts of risk factors and healthy lifestyles as methods of preventing disease. A risk factor is anything that increases the riskof disease in an individual. Lifestyle refers to the individual's personal behaviors with regard to risk factors. Identifying risk factors and modifying them by changing lifestyles in order to prevent disease has become ubiquitousas a strategy in public health. The book examines the history and evolution of the concepts of risk factors and healthy lifestyles and their application to coronary heart disease, the major chronic disease of the twentieth century. The first part contains a history of the use of statistics in public health and medicine, and the ways in which various industries developed the concept of the risk factor. The second part describes the concept of healthylifestyles, which was devised by municipal public health departments and life insurance companies in the early part of the century. The third and fourth parts examine how the concepts of risk factors and lifestyles were applied tothe primary chronic disease of the twentieth century -- coronary heart disease. The focus of the book overall is on coronary heart disease as a public health, rather than a medical, issue, and the various concepts that have beenused in preventing it. William G. Rothstein is Professor of Sociology at the University of Maryland, Baltimore County.