Obesity can be defined as the excessive accumulation of fat in adipose tissue, to the extent that health may be impaired. The most widely used measures of total and abdominal adiposity are the body mass index and waist circumference. Obesity is now a global public health problem, with about 315 million people world-wide estimated to fall into the WHO-defined obesity categories with a body mass index (BMI) of 30 or above. The primary causes of the rapid global rise in obesity rates lie in the profound environmental and societal changes now affecting large parts of the world and creating societies in which physical activity is low and the availability of high-fat, energy-dense foods has increased. Strategies aimed at preventing weight gain and obesity have not been successful to date but are likely to be more cost effective, and to have a greater positive impact on long-term control of body weight than treating obesity once it has developed.