Training stress in the absence of adequate recovery has been associated with a decrease in well-being and performance. Thus, there is potential for the high training and competition loads that elite English youth football players experience to have a negative effect on wellbeing and performance. The aim of the thesis was to assess the utility of well-being and physical performance assessments in managing the development of elite English youth football players. The first study (Chapter 4) examined the sensitivity of a subjective well-being questionnaire (WQ; developed ‘in-house’ by sport science practitioners at a category two academy and only taking < 30 s to complete), by comparing the player’s next day responses between two acute training bouts of varied duration; 15 mins (low load) compared to 90 mins (high load) high intensity intermittent exercise (Loughborough intermittent shuttle test, LIST). WQ items showed small to large deteriorations following the high load compared to low load (d=0.4-1.5, P=0.03-0.57). The ability of the WQ to differentiate between responses to high and low training loads indicated that this questionnaire could be used to detect training induced stress prior to training on a daily basis throughout the season. Other modes of monitoring assessment evaluated were either not sensitive to differentiate between high and low loads (countermovement jump; CMJ) or detected differences between high and low training load responses (HR indices) but lacked utility in detecting individual changes. The second study (Chapter 5) applied well-being and physical performance assessments to elite English youth football players during a high intensity, low volume pre-season training period. Trivial changes in perception of WQ items of sleep, recovery, appetite, fatigue, stress and muscle soreness were observed across weeks (P=0.35-0.93, 2 P  =0.02-0.08) with no negative WQ responses evident. Internal training load was lower to a large extent in week 1 (P= < 0.001, 2P  =0.54) yet no differences in internal training load were evident across weeks two, three, four and five. Trivial to small associations (r=-0.21 to 0.19) between internal training load and WQ responses were observed. Small to moderate improvements in aerobic performance were evident post training in comparison with pre training (P < 0.001-0.53, d= 0.33 – 0.94) with a large to moderate improvement in submaximal HR measures (P < 0.001 – 0.09; 2 P  = 0.34 - 0.74) observed across the training weeks. Trivial to moderate impairments in neuromuscular performance were evident post training in comparison with pre training (P < 0.001 – 0.21; d=0.17 – 1.00). Collectively, the preservation of well-being prior to each training session during a pre-season period and improvements in aspects of physical performance were indicative of a balance between stress and recovery. The third study (Chapter 6) examined player perceptions of well-being and physical performance across a season in Elite English youth football players. Increases in training exposure (P < 0.05; 2 P  =0.52) and moderate to large deteriorations in perceptions of well-being (motivation, sleep quality, recovery, appetite, fatigue, stress, muscle soreness P < 0.05; 2 P  =0.30-0.53) were evident as the season progressed. A large improvement in Yo-Yo intermittent recovery test performance (Yo-Yo IRT; P < 0.05; 2 P  =0.93) and a small to moderate impairment in neuromuscular performance (P > 0.05; 2 P  =0.18 - 0.48) was observed as the season progressed. These findings show an imbalance between stress and recovery in English elite youth football players even when players experienced lower training exposure than stipulated by the elite player performance plan (EPPP). In summary, this thesis highlights the potential utility of subjective well-being assessments to inform the management English elite youth football player development. Furthermore, it highlights the high training volumes that English elite youth players are exposed can potentially lead to an imbalance between stress and recovery.