The research within this book answers four major questions: 1) Do elite hockey players arrive for a game hydrated and do they consume enough fluid to prevent dehydration over the course of a game? 2) Is hydration status repeatable between days and can an athlete who arrives dehydrated prior to training or competition become hydrated in the time before the start of activity? 3) What is the extent of dehydration (%body mass (BM) loss) necessary to change substrate oxidation and skeletal muscle metabolism during exercise in male and female subjects? 4) Will progressive dehydration have a negative effect on endurance performance?