The List of Prohibited Substances and Methods (the List), an International Standard published yearly by the World Anti-Doping Agency (WADA), determines which substances and methods are prohibited in sport in- and out-of-competition. Stimulants are included within drug class S.6 under the in-competition testing section of the List. Athletes may be tempted to use stimulants as ergogenic aids in-competition in order to temporarily improve their mental and/or physical functions by increasing alertness, aggressiveness, motivation, locomotion, heart rate, and reducing fatigue. The Prohibited List Expert Group, responsible for the maintenance of the List, approved WADA funding for a two-year study to determine whether athletes were also using stimulants to benefit from their performance-enhancing effects during the training phase between competitions (i.e., out-of-competition). This study, involving 11 WADA-accredited laboratories, found that the use of stimulants by athletes during training was not significantly prevalent (0.36% of positive findings), suggesting that this issue does not, at the moment, pose a further challenge to the fight against doping in sport. In addition, the study supports the current structure in the Prohibited List that differentiates banned substances into the in- and out-of-competition classifications.