Gastroenteritis is inflammation of the stomach and intestines; colitis is inflammation of the colon. Viruses are the most common cause, followed by bacteria and parasites. Incidence of the various infections varies by age, sex, location, and vaccine availability; vaccination has reduced rotavirus infections by as much as 90% in children. Postinfectious complications include irritable bowel syndrome (IBS) and lactose intolerance. Approximately 9% of patients with acute gastroenteritis or colitis develop postinfectious IBS, which accounts for more than 50% of all IBS cases. The diagnostic approach to gastroenteritis and colitis varies with symptom severity. Microbial studies are not needed with mild symptoms that resolve within a week, but longer-lasting or more severe symptoms (including bloody stool) warrant microbial studies. In addition, recent antibiotic exposure should prompt testing for Clostridioides difficile. Multiplex antimicrobial testing is preferred; stool cultures and microscopic stool examinations are no longer first-line tests. Management depends on severity. Patients with mild or moderate symptoms are treated with oral hydration if tolerated; nasogastric or intravenous hydration are used for those with more severe illness. In addition, antiemetic, antimotility, and/or antisecretory drugs can be used for symptom control. Antimicrobial therapy is indicated for C difficile infections, travel-related diarrhea, other bacterial infections with severe symptoms, and parasitic infections.