Athletes and physically active people consume many ingredients with purported ergogenic effects. Some of these components may interact negatively with the major, conventional ingredients in sports drinks. This chapter discusses only those that have been studied in association with the rehydration properties of beverages. It demonstrates the effect of potassium concentrations in beverages on diuresis. Nutritional products with protein or combinations of amino acids represent an important share of all supplements used by athletes. Caffeine is widely used as an ergogenic aid and is often present in the regular diet of athletes and sedentary individuals. The chapter discusses the acute rehydration characteristics, with emphasis on the role of caffeine on post-exercise rehydration. It provides information on natural drinks, including their acceptability and composition. There is good experimental evidence to support skim milk as an effective beverage for rehydration after exercise in the heat, both in children, adolescents, and adults.