Lombard, WP, Durandt, JJ, Masimla, H, Green, M, and Lambert, MI. Changes in body size and physical characteristics of South African under-20 rugby union players over a 13-year period. J Strength Cond Res 29(4): 980–988, 2015—This study compared changes in the body size and physical characteristics of South African under-20 rugby union players over a 13-year period. A total of 453 South African under-20 players (forwards: n = 256 and backs: n = 197) underwent measurements of body mass, stature, muscular strength, endurance, and 10- and 40-m sprint times. A 2-way analysis of variance was used to determine significant differences for the main effects of position (forwards vs. backs) and time (1998–2010). The pooled data showed that forwards were significantly heavier (22%), taller (5%), and stronger (18%) than the backs. However, when 1 repetition maximum strength scores were adjusted for body mass, backs were stronger per kg body mass. Stature did not change over the 13-year period for both groups. There were, however, significant increases in muscular strength (50%), body mass (20%), and muscular endurance (50%). Furthermore, an improvement in sprint times over 40 (4%) and 10 m (7%) was evident over the period of the study. In conclusion, the players became heavier, stronger, taller, and improved their upper-body muscular endurance over the 13 years of the study. Furthermore, sprint times over 10 and 40 m improved over the same time period despite the increase in body mass. It can be speculated that the changes in physical characteristics of the players over time are possibly a consequence of (a) adaptations to the changing demands of the game and (b) advancements in training methods.