In this paper I have defined obesity and indicated its prevalence, as well as its risks. Body fat and its relation to other body components can be quantitated in many ways. From a practical point of view, the use of body mass or Quetelet index, defined as the ratio of weight (kg) divided by the square of the height (m2) is the most useful. Overweight is defined as a BMI of 25 to 30 kg/m2 and obesity as a BMI above 30 kg/m2. The WHR can provide additional information about the risk of obesity. Using BMI, the prevalence of overweight in the English-speaking countries of Australia, Great Britain, and the United States is almost identical at 24% of women and 31 to 34% of men. In the obese category, there are more Americans (12%) than in the other two countries (6-8%). There is a U-shaped relationship between weight and risk of death. When body weight is increased 20% above average, the extra mortality rises to 20% for men and 10% for women. This extra mortality is associated with an increased death rate from heart disease, hypertension, diabetes mellitus, digestive diseases, and cancer. In addition to an increased risk of death, overweight individuals demand more from their heart, lungs, and musculoskeletal and digestive systems.