Eighty per cent of all gymnasts are younger than 18 years with the vast majority aged between 9 and 16 years. The balance between the contributions of the physiological variables underpinning gymnastics routines therefore varies not only according to the length and demands of particular routines but also the age, growth, maturation, sex, and state of training of the gymnasts. This chapter analyses the energy demands of gymnastics routines in the context of developmental exercise physiology. Current concepts of exercise metabolism, aerobic fitness, high-intensity exercise performance, and muscle strength in relation to sex, growth, age, and maturation are examined. The physiological principles underlying gymnastics training are discussed and the phenomenon of the unexplained underperformance syndrome during childhood and adolescence is outlined. Challenges for further research are identified.