Phibbs, PJ, Roe, G, Jones, B, Read, DB, Weakley, J, Darrall-Jones, J, and Till, K. Validity of daily and weekly self-reported training load measures in adolescent athletes. J Strength Cond Res 31(4): 1121–1126, 2017—The primary aim of the study was to assess the level of agreement between the criterion session rating of perceived exertion (sRPE30min) and a practical measure of a self-reported Web-based training load questionnaire 24 hours after training (sRPE24h) in adolescent athletes. The secondary aim was to assess the agreement between weekly summated sRPE24h values (∑sRPE24h) and a weekly Web-based training diary (sRPEweekly) for all field-based training accumulated on a subsequent training week. Thirty-six male adolescent rugby players (age, 16.7 ± 0.5 years) were recruited from a regional academy. Measures of sRPE30min were recorded 30 minutes after a typical field-based training session. Participants then completed the sRPE24h via a Web-based training load questionnaire 24 hours after training, reporting both session duration and intensity. In addition, on a subsequent week, participants completed the sRPE24h daily and then completed the sRPEweekly at the end of the week, using the same Web-based platform, to recall all field-based training session durations and intensities over those 7 days. Biases were trivial between sRPE30min and sRPE24h for sRPE (0.3% [−0.9 to 1.5]), with nearly perfect correlations (0.99 [0.98–0.99]) and small typical error of the estimate (TEE; 4.3% [3.6–5.4]). Biases were trivial between ∑sRPE24h and sRPEweekly for sRPE (5.9% [−2.1 to 14.2]), with very large correlations (0.87 [0.78–0.93]) and moderate TEE of 28.5% [23.3–36.9]. The results of this study show that sRPE24h is a valid and robust method to quantify training loads in adolescent athletes. However, sRPEweekly was found to have a substantial TEE (28.5%), limiting practical application.