Power is regarded as a major attribute for most team sport players, therefore it’s advancement should be taken into consideration early in an athlete’s developmental pathway. However, the natural development of power and its trainability during growth and maturation remains relatively unaddressed by researchers. Subsequently, the purpose of this thesis was to investigate the variability of isoinertial force-velocity profiling and maximal strength assessment in youth and determine the role of maturation and training on the variables deemed reliable. A great deal of movement variability in the eccentric phase of counter movement jumps was found, especially prior to peak height velocity (PHV). Vertical concentric mean and peak power and eccentric mean power were deemed reliable to monitor stretch-shortening cycle performance, while the assessment of force-velocity-power and maximal strength can be measured reliably with a ballistic loading protocol after familiarization. Vertical and horizontal power can be normalised to body mass with a common sex allometric scaling factor in athletes between 9-12 years old and sex difference in relative acyclic power did not appear prior to male puberty in contrast to sprint and change of direction performance. Tracking somatotype, age, maturity and body mass during growth was found important to better understand the development of explosive actions. In male pre to post PHV, strength and power measures were found to have a greater dependency on body mass than velocity-related variables, and even after adjustment for body mass most differences remained substantial. Maturity dependant improvements, along with appropriate adjustment for body mass, need to be taken into account when comparing performance of maturing athletes. Percent differences in strength or power had percent effects on sprint performance that were similar at different stages of maturity. These relationships explained most of the maturity related improvements in sprint performance before PHV but only some improvements after, meaning that other neuromuscular factors are playing a role in sprint performance development. Specific strength training is recommended for athletes around PHV rather than prior to PHV to improve strength, power and speed, while maintenance programs in youth should be planned to reduce decay in these variables, especially in pre and post PHV boys. While the development of power is dependent on a multitude of factors, maturity had an influence on the timing and tempo of power development as well as the training dose-response relationship and rate of decay.