General considerations in assessing body composition in children and youths are described. Various methods are explored and recommendations are made for methods to be used in the International Growth Standard for Preadolescent and Adolescent Children Project. Exclusion of under- and overweight participants is recommended, and a method is proposed to assess both underweight and overweight. In addition to height and weight, we recommend waist circumference, selected skinfolds, and dual-energy x-ray absorptiometry (DXA) as a measure of fat, lean, and bone mineral density. We also propose using both fat mass index and fat-free mass index as an improvement over body-mass index.