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Dehydration, or the loss of fluid and salt from the body, can be an uncomfortable and sometimes life-threatening disorder. In the healthy individual, dehydration may occur as a result of vigorous exercise, after exposure to extreme degrees of heat, or in industrial workers working in warm to hot ambient temperatures with protective clothing. The degree of dehydration determines the occurrence of symptoms and complications. Low levels of dehydration (2% loss of body weight or more) impair cardiovascular and thermoregulatory responses and reduce the capacity for further work or exercise (1). Dehydration may also occur as a result of illness characterized by excessive loss of fluid from the body, such as in diarrhea or in diseases that lead to excessive vomiting or excessive loss of water in the urine. In these conditions, water loss may sometimes exceed 10% of body weight, and the disturbance of homeostasis may become life-threatening. Indeed, dehydration from diarrhea is a significant cause of morbidity and mortality in children in developing countries. Rehydration refers to the replacement of fluid and electrolytes to correct dehydration. In severe dehydration, particularly when there is ongoing fluid loss, as in diarrhea, rehydration must be achieved intravenously. In less severe dehydration (and in the absence of significant vomiting), the gastrointestinal (GI) tract has the capacity to absorb fluid rapidly and can be the route of rehydration. This chapter focuses exclusively on oral rehydration and briefly examines recent progress in the understanding of the mechanisms underlying intestinal absorption of fluid and electrolytes and how this understanding affects the design of beverages intended for oral rehydration.