Assessment of body and weight gain composition is one of the major keys for the evaluation of nutritional requirements in preterm and term infants. The changes in body composition during the first years of life seem to play an important role in the nutritional programming of adult morbidity. Noninvasive and indirect techniques are the most suitable methods for measuring body composition in preterm and term infants. Most of these techniques are used in clinical practice, whereas others are used mainly for research.