Acute gastroenteritis is defined as a diarrheal disease of rapid onset, with or without nausea, vomiting, fever, or abdominal pain. In the United States, acute gastroenteritis accounts for 1.5 million office visits, 200,000 hospitalizations, and 300 deaths in children each year. Evaluation of a child with acute gastroenteritis should include a recent history of fluid intake and output. Significant dehydration is unlikely if parents report no decrease in oral intake or urine output and no vomiting. The physical examination is the best way to evaluate hydration status. The four-item Clinical Dehydration Scale can be used to determine severity of dehydration based on physical examination findings. In children with mild illness, stool microbiological tests are not routinely needed when viral gastroenteritis is the likely diagnosis. Mild gastroenteritis in children can be managed at home. Oral rehydration therapy, such as providing half-strength apple juice followed by the child's preferred liquids, is the mainstay of treatment for mild dehydration and is as effective as intravenous rehydration for preventing hospitalization and return to the emergency department. Oral rehydration solutions are recommended for moderate dehydration. Ondansetron may be prescribed if needed to prevent vomiting and improve tolerance of oral rehydration solutions. Hospitalization and intravenous fluids are recommended for children who do not respond to oral rehydration therapy plus an antiemetic and patients with severe dehydration (i.e., signs of shock or more than 10% dehydration). Handwashing, breastfeeding, and rotavirus vaccination reduce the incidence of acute gastroenteritis in young children.