Speranza, MJA, Gabbett, TJ, Greene, DA, Johnston, RD, and Sheppard, JM. Changes in rugby league tackling ability during a competitive season: the relationship with strength and power qualities. J Strength Cond Res 31(12): 3311–3318, 2017—This study examined the relationship between changes in tackling ability, and muscular strength and power during a semiprofessional rugby league competitive season. Twelve semiprofessional rugby league players (mean ± SD age, 23.3 ± 2.0 years) underwent tests of upper- and lower-body strength and power during the preseason period. Tackling ability was tested using video analysis of a standardized one-on-one tackling drill. Players repeated these tests after round 15 of a 25-match competitive season. Changes in 1 repetition maximum (1RM) squat (rs = 0.70; p < 0.02) and squat relative to body mass (rs = 0.73; p < 0.01) were significantly related to changes in tackling ability. Players with the greatest improvements in tackling ability (i.e., “responders”) retained 1RM squat (effect size, ES = 0.85, p = 0.09) and squat relative to body mass (ES = 0.82, p = 0.15) to a greater extent than the “nonresponders.” The results of this study suggest that players who retained lower-body strength were able to improve tackling ability during the competitive season, whereas reductions in lower-body strength were associated with decrements in tackling ability. This study highlights the importance of the development and maintenance of lower-body muscular strength for effective tackling performance throughout the rugby league season.