An agreed definition of obesity as a body mass index (BMI) of 30 kg/m2 or more seems to be accepted everywhere except in North America. Recent data confirm the importance of setting an upper individual BMI limit of 25 kg/m2 and a population optimum of 20-23 kg/m2. Some adjustment of BMI should be made in individuals and populations with disproportionate shapes, e.g. short or long legs, and morbidity and mortality risks are especially important in those with a waist measurement of about 102 cm or more, the risk increasing from 88 cm. Waist measurements should probably now be substituted for the waist/hip circumference ratio. Diabetes is universally closely linked to increases in BMI, and cardiovascular disease is amplified by obesity, particularly in western societies where other dietary factors contribute substantially. Industrialization with reduced physical activity and higher fat diets lead to obesity first in middle-aged women, then in men, with younger adults and children eventually being affected. Physiological studies display the interaction of physical activity and energy dense, high fat diets and explain the secular, age- and social class-related trends throughout the world. Intergenerational amplification of obesity may be underway, so the public health implications of obesity are immense.