The natural process of growth and maturation encompasses a host of physiological and hormonal changes. This in turn has a profound influence on the young athlete’s capacity for aerobic and anaerobic metabolism, as well as impacting upon neuromuscular coordination factors, which alter efficiency and economy of locomotion (Boisseau and Delamarche, 2000). In addition to influencing the young athlete’s capacity for different forms of training, the phase of growth and maturation of the athlete will also dictate the mechanisms through which training effects occur (Stratton et al., 2004). As with other forms of training, what constitutes appropriate metabolic conditioning for a young athlete will be determined to a large degree by their age and stage of physical development (Gamble, 2009b). With respect to metabolic conditioning, this pertains to both the energy systems involved and also what form metabolic conditioning takes, in terms of the mode and format of training employed. Popular long-term athlete development models advocate that the nature of endurance training should differ markedly for young athletes at the different stages of development (Balyi and Hamilton, 2004). The rationale for this approach will be critically examined in this chapter.