The only indicator of development that is available from birth to maturity is skeletal age. This short review discusses how standard bone ages have been developed from assessment of radiographs, and describes the advantages and disadvantages of the 'atlas' approach as developed by Greulich and Pyle, and the bone by bone approach, as developed by Tanner. As the standards currently available are based mainly on historical series of radiographs from particular populations, it is stressed that national standards should be established and updated regularly if bone ages are to be used to assess development. The question of the clinical relevance of using bone age assessments of the hand and wrist to determine the state maturation of the whole skeleton and particularly, the growth potential is also discussed. It is concluded that, despite the difficulties of assessing bone age, and the assumptions on which the various methods are based, determination of skeletal development is clinically relevant in that it provides the only means of assessing rates of maturational change throughout the growing period.