Exercise-induced changes in muscular and cardiovascular function in pre- and postpubescent children are explained in terms of a "Trigger Hypothesis." This hypothesis predicts that, prepubertally, there will be only small training-induced biological alterations because of the lack of hormonal control. It is suggested, therefore, that emphasis be placed on skill acquisition rather than physiological conditioning during prepuberty. Postpubertal exercise-induced changes are well documented and follow predictable patterns. The principles that govern physiological adaptations to exercise are discussed in terms of energy transfer and the factors that affect training. Duration, intensity, and frequency of performance are detailed. It is recommended that emphasis be placed on these factors when designing a physiologically sound physical training program.