Obesity is a serious medical problem that affects millions of people, especially in Western societies. Although long considered a complicating factor in a variety of diseases, there is now widespread agreement that obesity itself should be classified and treated as a disease and that it has important conse quences for personal health, quality of life and cost to society. Understanding obesity and the means of treating it have been hampered in the past. There have been misperceptions that obesity is a behavioral disorder and that its treatments provides only cosmetic benefits. Pharmacologic approaches to treatment have suffered from problems of limited efficacy, reduced activity upon chronic use, and serious side effects, including abuse liability, cardiac disease, hypertension, and respiratory complications. Finally, there has been a proliferation of consumer and natural products with unproven benefits. This book attempts to address both the problems associated with obesity and the approaches to treating it. In the first section devoted to pathology, Drs. DIGIROLAMO, HARP, and STEVENS elaborate in Chap. 1 on how obesity and its medical complications develop. As described by Dr. PI-SUNYER in Chap. 2, obesity is a disease seen most often in affluent Western societies and is associated with the aforemen tioned medical problems, as well as Type II diabetes mellitus and gallbladder disease. Drs. CHAGNON, PERUSSE, and BOUCHARD review the human genetics of obesity in Chap. 3, and Drs.