The aim of this thesis was to develop and validate squash-specific fitness tests to identify physiological determinants of repeat-sprint ability and performance in sub-elite and elite-standard squash players. Study one examined the validity of a squash-specific test of endurance capability and VO[2max]. Trained squash players and runners completed squash-specific and treadmill incremental tests to exhaustion. Squash players achieved greater VO[2max] on the squash-specific than the treadmill test while runners did not differ. Squash players exercised longer than runners on the squash-specific test despite similar VO2max. The squash test discriminated endurance capability between squash and non-squash players of similar fitness and elicited higher VO[2max] in squash players. The results suggest that it is a valid test of aerobic fitness in squash players. Study two assessed the reproducibility of physiological and performance measures from the squash specific test in county-standard players. Test-retest variability was low for all measures (Typical error < 5%) though the magnitude varied depending on the metric used. Studies three and four examined the validity and reproducibility of squash-specific tests of change-of-direction speed and multiple-sprint ability. County-standard squash players and footballers completed squash-specific and equivalent non-specific tests on separate days. Performance time was recorded. Participants repeated the tests seven days later to assess reproducibility. Squash outperformed non-squash players on the squash-specific tests despite similar non-specific capabilities. Squash-specific tests discriminated squash player rank while non-specific tests did not. Test-retest variability was low (Typical Error < 3%) for both tests. Squash-specific tests predicted ability in squash players and discriminated between squash and non-squash players of equal nonspecific fitness. The findings suggest that the squash-specific tests are valid for the assessment of high-intensity exercise capabilities in squash players. Studies five and six explored correlates of multiple-sprint ability and performance in sub-elite and elite squash players. Squash-specific and general tests were performed by regional league players ranging from division three to premier standard and elite players on three tiers of a national performance program. In sub-elites, multiple-sprint ability and endurance capability discriminated performance and multiple-sprint ability was related to change-of-direction-speed, VO[2max] and endurance capability. In world-ranked men and women, the ability to perform and sustain rapid changes of direction correlated with multiple-sprint ability and together with multiple-sprint ability discriminated performance. Aerobic fitness was not related to performance or multiple-sprint ability in elite players. Senior elites performed better than players on the talented athlete scholarship scheme (TASS) on all tests except VO[2max] and counter-movement jump. Drop-jump power and reactive strength discriminated senior and transition level from TASS players and indices from the multiple-sprint test discriminated seniors from transition and TASS players. This thesis has validated squash-specific tests of endurance and high-intensity exercise capabilities. These tests have shown that high-intensity exercise capabilities determine performance in elites while sub-elite performance is determined by multiple-sprint ability and endurance capability. The findings can be used to improve assessment of training effects and to inform the design of effective training methods.