Madsen, CM, Badault, B, and Nybo, L. Cross-sectional and longitudinal examination of exercise capacity in elite youth badminton players. J Strength Cond Res 32(6): 1754-1761, 2018-Badminton-specific speed and endurance performance was evaluated in both cross-sectional and longitudinal studies of elite youth players and compared with the physiological capacities of world top-50 singles players. The cross-sectional study involved 10 males in the category U15 (<15 years), 10 U17, 9 U19, and 4 senior elite players. They performed 30-m sprint, countermovement jump, and badminton-specific speed (B-SPEED) and endurance (B-ENDURANCE) tests. The longitudinal data were collected for 10 U15 players with 1- and 2-year follow-up measures. Compared with seniors, B-SPEED performance was 20 ± 5% slower for U15 (p < 0.001), 7 ± 3% slower for U17 (p < 0.05), and nonsignificantly slower (3 ± 3%; p = 0.27) for U19. B-ENDURANCE performance was 45 ± 11% shorter for U15 (p < 0.001), 25 ± 7% shorter for U17 (p < 0.001), and 17 ± 10% shorter for U19 (p < 0.01). The longitudinal data for U15 revealed that B-SPEED performance improved from a 19 ± 5% (p < 0.001) deficit at baseline to 5 ± 5% (p = 0.23) at first-year, and 2 ± 3% (p = 0.43) at second-year follow-up. B-ENDURANCE performance improved from a 43 ± 11% deficit at baseline to 27 ± 10 and 17 ± 7% at 1- and 2-year follow-up (p < 0.001), respectively. Countermovement jump also improved with aging from 29 ± 5% deficit for U15 to 13 ± 6% deficit for U19 (p < 0.01). In conclusion, B-SPEED improves markedly with aging in youth elite players to achieve, by age 19, values matching world-class players. Endurance improved markedly, but with a significant deficit remaining in comparison with senior elite players.