To some extent body composition reflects nutritional status. It is also influenced by age, sex, race, physical activity and disease. The method used to measure body composition depends on the variable to be quantified. It may also depend on the practical conditions of the study. Detailed methods, such as densitometry, isotope dilution and electrical impedance give more accurate information, but they are commonly based on hypotheses established in adults. Anthropometric measurements can be used directly or as ratio or regression equations. At adolescence, the weight/height<sup>2</sup> or body mass index (BMI) is preferred to weight for height as age is taken into account. In addition, the BMI pattern reflects real changes in body shape, and early in life it is an indicator of later development. In addition to measuring weight and height skinfold (SF) measurements should be carried out. The triceps SF is usually recommended and widely used as it is better than the subscapular SF to predict percent body fat. Trunk SFs, such as the subscapular, are better than extremity SF for their association with internal fat and their good correlations with risk factors and response to nutritional interventions. Chemical changes in the body are observed during growth. Chemical maturity does not occur until after puberty, but most changes occur early in life. Tracking is the maintenance of an individual in the same percentile range across age and varies according to the growth parameter and to the period of growth. Low tracking of fatness (up to the age of 8 years) corresponds to the period of rapid chemical changes. Low tracking of stature is observed at adolescence when height velocity is high. Nutrition affects fatness and stature, but the consequences of under- and over-nutrition differ between early childhood and adolescence. BMI curves show that most changes have their origin during the first years of life.