Obesity has been evident in the human record for over 20 000 years and affected numerous aspects of human life and society (Bray, 2007a; Bray, 2007b). This introductory chapter describes the early history of human obesity, and then reviews how understanding has developed in the basic biology of obesity, its defi nitions and measurement, the complications of the disease, and fi nally its management. Some of the major scientifi c and medical milestones in the history of obesity are shown in Table 1.1.