Athletes who participate in sports where performance relies on repeated high-intensity efforts could benefit from creatine (Cr) ingestion due to an increased ability to perform and recover from high-intensity exercise bouts either during training or competition. However, few studies exist which have investigated the effects of acute short-term Cr supplementation on appropriately simulated soccer-specific performance.Aims. To determine the reproducibility of a 90 minute soccer-specific performance test and to subsequently examine the effects of acute short-term Cr ingestion (1 week) on soccer-specific physical performance. Study design. Two experimental designs were adopted for this thesis. For study one, a test-retest design was used to determine the reliability and validity of the Ball-sport Endurance And Sprint Test (BEAST). Two trials of the BEAST were performed, separated by five to seven days. For study two, a randomised, triple-blind, placebo-controlled experimental design was adopted to determine the efficacy of acute short-term Cr supplementation (seven days) on soccer-specific performance, using the BEAST protocol.Methods. Twenty male amateur soccer players volunteered to participate in the study. For study one, the test-retest reliability of several soccer-specific performance measures obtained during a modified version of the BEAST was quantified using the standard error of measurement (Van Cutsem, Duchateau, & Hainaut) (or typical error) (Hopkins, 2000), coefficient of variation (CV), and Intraclass Correlation Coefficient (ICC). For study two, the cohort was split and subjects randomly allocated to one of two groups (Cr supplementation and Placebo) on a matched-pair basis. The Cr group (mean age 25.4 ± 4.5 years, mean body-mass 79.3 ± 10.5 kg) ingested 20 g of Cr and 8 g of glucose powder per day for seven days, whereas the placebo (mean age 26.7 ± 4.6 years, mean body-mass 80.8 ± 8.6 kg) group ingested 20 g of corn-flour and 8 g of glucose per day for seven days. The effects of acute short-term Cr supplementation were analysed by repeated measures ANOVA. In addition, effect sizes (ES) were calculated and entered with the associated p-value into Hopkins' spreadsheet for determination of the ES confidence limits (95%) and the chances that the true effect was substantial (i.e. ES ≥ 0.2). Clinical/practical inferences were made accordingly.Results. Study 1: The BEAST protocol had good reliability (high ICC values, relatively low coefficients of variation, low noise to signal ratios) and face validity (HR, VO2, distances covered, duration, and movements performed in the BEAST were all similar to those reported in actual soccer matches). Study 2: Performance of the four major physical measures (12 m sprint, 20 m sprint, circuit time and vertical jump) during the BEAST deteriorated during the second half relative to the first half for both Cr and placebo groups, indicating a fatigue effect associated with the protocol. HR and body-mass values also decreased for both groups during the 90 minute protocol. However, there was no statistically significant differences between the groups for these four measures or for body-mass, HR or VO2max values, suggesting Cr had no substantial effect (relative to placebo) on improving physical performance (or reducing fatigue). When the effects were assessed for the whole 90 minute BEAST protocol, all effects showed a negative trend and, correspondingly, the chances of a detrimental effect were greater than the chances of a beneficial effect.Conclusions: The 90 minute BEAST protocol had good reliability and face validity making it a suitable soccer simulation and performance protocol with which to investigate the effects of Cr supplementation on soccer performance. However, no significant (statistical or clinical) effects of acute short-term Cr supplementation on soccer performance were observed suggesting its potential use as an ergogenic aid for soccer players is questionable.