Commonly used indicators of biological maturation are discussed, including sexual, skeletal, morphological, and dental maturity, and the hypothalamus-pituitary-end organ axes that regulate the growth and maturation processes. Interrelationships among indicators and the tempo, timing, and sequence of maturational events are also considered. Environmental factors that influence the level of maturity at a given point in time and the process of maturation are also discussed: undernutrition, obesity, ethnic/racial background, social class, familial characteristics, climate, and altitude. Recommendations for the design of studies of maturational events are made, and an overview of secular changes before and after 1970 is provided. The review concludes with specific recommendations for the inclusion of a maturity indicator or maturity indicators in the construction of an international growth standard for preadolescent and adolescent children