Considering the high prevalence and the increasing trends, obesity is now considered as a public health problem in numerous countries. The main aim of the National Program of Nutrition and Health is to stop the increasing prevalence of childhood obesity. In this frame work, a group of experts has established a new presentation of the corpulence curves, adapted for clinical practice, to define normal weight and obesity. Weight status is now currently assessed on the basis of weight and height measurements, after computing the Quetelet index or body mass index (BMI) corresponding to weight (m) divided by square of height (weight/height2). As body proportion varies during growth, age must be taken into account. Various curves were published. In 1982, based on the French sample of the international growth study, BMI curves were published. They were revised in 1991. The third and 97th centiles define the normal weight range. Overweight is defined by BMI values greater than the 97th centile. In the year 2000, a new international definition was established. Two centiles were constructed to define overweight and obesity. The new BMI charts adapted for clinical practice, proposed by the French National program of nutrition and health, include the French reference curves plus the centile defining obesity in the international definition. Thus, in the new French charts, the area above the 97th centile is split in two levels (degree 1 obesity and degree 2 obesity). Drawing the BMI curve for each child, like drawing weight and height curves, is a simple act which can be done routinely. The age at adiposity rebound (an indicator predicting the risk of adult obesity) can be read from the curve. It allows to identify an early phase of obesity development, even at the time when overweight is not yet clinically visible. When obesity appears clearly, the identification is easy. The use of BMI curves is particularly useful in two situations: (1) in very young overweight children, the curves allow to identify children who have a real risk of developing obesity. (2) By the age of 6 years, when due to normal physiological variations, clinical assessment can be misleading. The BMI curves allow to identify children at risk. When a child is identified as having a real risk of obesity, simple preventive measures, adapted for each subject, could avoid a development toward massive obesity, which may become difficult to reduce if managed too late.