Martínez-López, EJ, De La Torre-Cruz, M, Suárez-Manzano, S, and Ruiz-Ariza, A. Analysis of the effect size of overweight in muscular strength tests among adolescents: reference values according to sex, age, and body mass index. J Strength Cond Res 32(5): 1404–1414, 2018—The aim of this paper is to quantify the effect size of overweight on the results of muscular strength tests in adolescents and to report percentile tables based on sex, age, and body mass index (BMI). The study hypothesized that the effect size obtained from the difference between normal-weight and overweight youth would be greater than the differences between sexes within the same age group. A total of 11,044 Spanish adolescents (48.5% girls) aged 14.39 ± 1.21 years (range: 12–16 years) from 42 secondary schools participated in the research. Muscular strength was evaluated using standing long jump, hand grip strength (manual dynamometer), and sit-ups (30 seconds). The effect size was analyzed using the adjusted Hedges' ğ. The results show that 76.3 and 72.8% of overweight boys and girls, respectively, performed a standing long jump equal to or less than the normal-weight average. The 67.4 and 67.1% of overweight boys and girls, respectively, showed manual dynamometer values equal to or greater than the normal-weight average. Finally, 68.7 and 65.9% of overweight boys and girls, respectively, obtained measures for 30 seconds of sit-ups equal to or lower than the normal-weight average. It can therefore be concluded that the differential effect size between boys and girls is higher than that between normal-weight and overweight adolescents in the 3 strength tests analyzed. Despite the above, these results suggest the value of taking into account the BMI when assessing the muscular strength of young people, in addition to sex and age.