Kilit, B and Arslan, E. Effects of high-intensity interval training vs. on-court tennis training in young tennis players. J Strength Cond Res 33(1): 188-196, 2019-This study aimed to examine the effects of 6 weeks of high-intensity interval training (HIIT) vs. 6 weeks of on-court tennis training (OTT) on the psychophysiological responses, performance responses, and technical scores of young tennis players. Twenty-nine young male tennis players (aged 13.8 ± 0.4 years) were divided into HIIT (n = 14) and OTT groups (n = 15). Both groups trained for the same total training duration with passive rest in each session. Pre-test and post-test included maximum oxygen consumption ((Equation is included in full-text article.)max), sprinting, jumping, 400-m running time, a tennis-specific technical test, and the t-drill agility test. The training interventions resulted in similar improvements in (Equation is included in full-text article.)max responses (HIIT: +5.2%, d = 1.36 [large effect]; OTT: +5.5%, d = 1.50 [large effect]). Both training protocols increased jumping and sprinting performances significantly from pre-testing to post-testing (p < 0.05, d values ranging from 0.40 to 1.10). The OTT group showed significantly higher performance responses in terms of the agility test performance and technical scores (p < 0.05, d = ranging from 0.77 to 0.88 [moderate effect]) compared with the HIIT group. By contrast, the HIIT group exhibited significantly higher performance responses in terms of the 400-m running time (p < 0.05, d = 1.32 [large effect]). Our results showed that tennis-specific on-court drills might be a more effective training strategy to improve agility and technical ability with greater physical enjoyment, whereas HIIT may be more appropriate for speed-based conditioning in young tennis players.