This chapter discusses the role that water, carbohydrate, electrolytes, and flavor play in fluid replacement and energy provision for athletes during/after exercise. Based on the scientific principles as well as some practical considerations, we conclude this chapter with recommendations related to beverage composition choices for athletes. Fluid replacement is a process that involves thirst and voluntary fluid intake, gastric emptying and intestinal absorption of fluid, distribution within the body fluid compartments, and whole-body fluid retention. The chapter focuses on the effects of beverage composition, particularly energy content and osmolality, which can significantly impact the rate at which gastric emptying and intestinal absorption of water occur. Carbohydrate-electrolyte beverages are typically formulated with the types and amounts of carbohydrates (or combinations of carbohydrates) that are rapidly digested and absorbed to facilitate delivery of fluid and carbohydrate to the body. Commercially available carbohydrate-electrolyte beverages typically contain glucose, sucrose, fructose, and maltodextrins.