Acute gastroenteritis is a leading cause of visits to physicians among children in the United States. Oral rehydration therapy has prevented or reversed dehydration among millions of children in developing countries. Although most U.S. health care providers are familiar with oral rehydration therapy, its proper use is still not widespread in industrialized nations. Viral pathogens in diarrheal illness can destroy absorptive cells at the intestinal villous tip while leaving secretory cells intact. Oral rehydration therapy takes advantage of the remaining intact absorptive cells, is less invasive than intravenous rehydration and allows parents to be involved in their children's care. This article outlines how to select patients for oral rehydration therapy, what fluids to use and how to implement therapy.