Aerobic fitness is important for most team sport players. However, the trainability of aerobic fitness using different approaches in developmental players, accounting for player maturation, is not well understood. Given the additional importance of technical and tactical skill acquisition for developing team sport players, the design, manipulation and quantification of responses and adaptations to small-sided games (SSG) could help reveal optimal training and prescription strategies for young players. Therefore, the purpose of this thesis was to design and determine appropriate training approaches for the development of aerobic fitness in young athletes. Firstly, a novel SSG was developed and thereafter a series of studies were undertaken to examine the effect of various SSG designs and training regimes on the physiological, movement and technical characteristics and performance variables in young players aged 12 to 14 years. Data revealed that players travelled further and at higher overall speeds, experienced higher physiological workloads, and performed more successful technical executions during non sport-specific SSG compared with a sport-specific equivalent. During the non sport-specific SSG, fewer player numbers (i.e. 3 vs. 3) provided a higher stimulus for aerobic fitness adaptation and improved technical executions compared with 4 vs. 4 and 6 vs. 6 games. Higher physiological loads were elicited during continuous 3 vs. 3 SSGs balanced for team selection and players travelled further at higher speeds during balanced games. Balanced and unbalanced team selections, and continuous and intermittent regimes, were interchanged without affecting the quantity and quality of technical executions. However, while manipulation of SSG rules reduced exercise intensity, the quality and quantity of technical executions was increased. The addition of inter-game high-intensity interval running elicited higher player external load and increased the distance travelled at higher running speeds. Perceptual response of players was influenced by external load of the various SSG formats more so than internal load. A combination of SSGs training and traditional high-intensity interval training was more effective at increasing aerobic fitness in young team sport players than SSGs training alone. In summary, non sport-specific, continuous SSG formats with small playing numbers, balanced team selection and no rule modifications appear to elicit the highest stimulus for aerobic adaptation and are therefore recommended for aerobic fitness development in young team sport athletes. Restricting the time allowed in possession of the ball could be used as a strategy to increase the technical skill capabilities of players, while intermittent regimes are recommend to train the associated demands of higher speed running. Finally, while SSGs can elicit sufficient stimulus for increasing aerobic fitness, the addition of high-intensity interval training to the training regime of young players around peak height velocity provides optimal development.