Introduction: To quantify changes in physical performance in men and women during British Army Junior Entry (Army-JE), Standard Entry (Army-SE), and Royal Air Force (RAF) Basic Training (BT). Design: Prospective longitudinal study. Methods: 381 participants [(339 men, 42 women) n=141 Army-JE; n=132 Army-SE; n=108 RAF] completed a 2-km Run, Medicine Ball Throw (MBT) and isometric Mid-Thigh Pull (MTP), pre- and post-BT. To examine changes in pre- to post- BT physical test performance, for each course, paired students t-tests, and Wilcoxon tests were applied to normally and non-normally distributed data respectively; with effect sizes reported as Cohen’s D and with rank biserial correlations, respectively. A one-way between-subjects ANOVA (or Welch ANOVA for non-normally distributed data) compared performance between quartiles based on test performance pre-BT. Where the main tests statistic, p value and effect sizes identified likely effect of quartile, post-hoc comparisons were made using Games-Howell tests with Tukey’s p value. Data are presented as mean ± standard deviation, statistical significance set at p<0.05. Results: During BT, 2-km run time improved by 13±46 (-2.1±8.1%), 30±64 (-4.8±12.3%), and 24±27 s (-4.5±5.1%) for Army-JE, Army-SE, and RAF, respectively (all p<0.005). MBT distance increased by 0.27±0.28 m (6.8±7.0%) for Army-JE (p<0.001) and 0.07±0.46 m (2.3±10.9%) for Army-SE (p=0.040), but decreased by 0.08±0.27 m (-1.4±6.0%) for RAF (p=0.002). MTP force increased by 80±281 N (10.8±27.6%) for Army-JE (p<0.001) and did not change for Army-SE (-36±295 N, -0.7±20.6%, p=0.144) or RAF (-9±208 N, 1.0±17.0, p=0.603). For all tests and cohorts, participants in the lowest quartile of pre-BT performance scores demonstrated greater improvements, compared with participants in the highest quartile (except Army-JE MBT; ∆% change similar between all quartiles). Conclusions: Changes in physical performance were observed for the three fitness tests following the different BT courses, but recruits with the lowest strength and aerobic fitness experienced greatest improvements.