There is increasing interest in measuring muscle mass in paediatric research and clinical practice. The simplest approach is to measure lean mass and predict muscle mass as a proportion of it, but individuals and groups vary in this ratio. Calculation of arm muscle area from anthropometry has been shown to have poor accuracy in individuals; segmental impedance analysis, however, may perform better as a simple predictive method for limb composition, and merits further development. Biochemical techniques (e.g. urinary creatinine measurement) are cumbersome in children and their data are confounded by variability in maturation, diet and activity level. The best tools are radiographic imaging techniques that attempt direct measurement of muscle mass. The most widely available is dual-energy X-ray absorptiometry, which is capable of detecting the direction of differences or changes, but is less reliable regarding their magnitude because of error that varies with gender, size and fatness. Magnetic resonance imaging is currently the best technique available, as computed tomography has too high a radiation dose to justify its use in younger age groups. Regardless of which technique is used, reference data are required to aid interpretation of results, and are a current research priority.