Body composition is increasingly demonstrated to be an important adult health outcome but receives little attention in paediatric clinical practice. There are several reasons why greater interest is merited. First, while obesity and eating disorders are currently defined by anthropometric criteria (weight relative to height, body mass index), these variables have poor sensitivity for monitoring response to treatment, and so body composition measurement could improve management. Second, body fat and its distribution merit monitoring more generally in patients in relation to the aetiology of cardiovascular disease, hypertension and type 2 diabetes, diseases now considered to have an “incubation period” during childhood and adolescence. Third, body composition is increasingly associated with clinical progress and outcome, including survival in some disease states. Finally, measurements of lean mass may improve the capacity to tailor nutrition, treatment and management to metabolic criteria.