The assessment of body composition in children has taken on greater significance because of the need to study the prevalence of obesity in children and youth, the need to better document the tracking and genetics of body fatness, the need to relate fat patterning in childhood and fat patterning in adults, and the need to assess changes in the prevalence in obesity over time in a given population. This paper reviews methods of estimating body composition in children and youth. The use of body mass index, anthropometry, body density, and bioelectric impedance methodologies is emphasized, as well as the need to use multicomponent models for validation studies of new methods. Also explored in this article is the relationship between body composition and health related fitness.