Introduction: Periodic testing is essential to evaluate changes in key physical and physiological characteristics in response to development strategies employed with adolescent rugby league players; however, the retest reliability of scientifically popular tests to assess these characteristics are yet to be comprehensively examined in this population. ...the aim of this study was to determine the retest reliability of the most commonly used tests in the literature, to assess key physical and physiological characteristics in adolescent, male rugby league players. Tests used to assess physiological characteristics demonstrated moderate to excellent ICC, with CV <5% including: linear speed, ICC=0.794, CV=1.35%; change-of-direction speed, ICC=0.787–0.860, CV=1.85–2.07%; aerobic capacity, ICC=0.854, CV=3.42%; muscular power, ICC=0.733–0.874, CV=3.72–4.02%; and muscular strength, ICC=0.944–0.988, CV=1.08–2.56%.