This chapter addresses issues related to being overweight and obese. In the past, obesity was considered simply as a state in which fat stores are visibly excessive, impairing health and quality of life. It was viewed largely as the result of excessive food consumption (gluttony) and lack of exercise (sloth), reflecting poor decision-making and lack of self-control. Body fat, at a degree that is excessive for an individual's age, sex, and race, impairs every aspect of health—physical, mental, social, and genomic. As such, obesity has become an exciting field of biomedical research, with more complete understanding of its biological basis. Both biological and environmental conditions are necessary to perturb the complex regulation of energy balance: in order to avoid weight gain and/or to lose weight, an individual must negotiate a powerful obesogenic environment. The chapter then looks at the measurement, clinical consequences, and management of obesity.