This chapter examines gastroenteritis, which, broadly defined, refers to any inflammatory process of the stomach or intestinal mucosal surface. However, the term usually refers to acute infectious diarrhea, a diarrheal syndrome of less than 2 weeks' duration, which may be accompanied by fever, nausea, vomiting, abdominal pain, dehydration, and weight loss. The acute infectious diarrheas can be divided into noninflammatory, inflammatory, and invasive. Most cases of acute gastroenteritis are self-limited and do not require medical attention. Physician consultation generally is advised for patients with a fever, dysentery (bloody stools), significant abdominal pain, dehydration, and risk factors for disease requiring intervention. Initial evaluation consists of the history, physical examination, and screening stool examination. Laboratory testing and antimicrobial therapy are recommended in a limited subset of patients based on this initial evaluation.