Obesity can be defined as the excessive accumulation of fat in adipose tissues, to the extent that health may be impaired. The most widely used measures of total and abdominal adiposity are the body mass index (BMI) and waist circumference, respectively. During the past several decades, the prevalence of obesity has increased substantially, making it a true epidemic and a public health crisis, with about 315 million people world-wide estimated to fall into the WHO-defined obesity categories with BMI of 30 or above. Indeed, the obesity epidemic has been already leading to dramatic increases in type 2 diabetes and metabolic syndromes. On the other hand, there are substantial risks for morbidity in obese children even before they reach adulthood, and if obesity in childhood persists into the adult years, the morbidity and mortality are greater than if the obesity developed in adulthood. The major 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 activities are low and the availability of high-fat, energy-dense foods has increased. Strategies aimed at preventing weight gain and obesity are likely to be more cost effective and to have a greater positive impact on the long-term control of body weight than treating obesity once it has developed.