In this installment of his serialized review of the Science of Hydration, Professor Tim Noakes considers the flawed science behind the American College of Sports Medicine’s 1987 Position Stand on the Prevention of Thermal Injuries During Distance Running. This Position Stand introduced the instruction to drink ahead of thirst during exercise to meet specific targets of fluid ingestion and reduce the risk of heat injury and dehydration—an instruction that eventually would prove to be fatal. Noakes also reviews scientific evidence regarding the human body’s innate fluid reserves and subsequent replacement needs.