A gastroenterologist is a physician who specializes in diseases of the digestive system, also called the gastrointestinal (GI) tract. Gastroenterologists have extensive training in the diagnosis and treatment of conditions that affect the esophagus, stomach, small intestine, large intestine (colon), and biliary system (e. G. Liver, pancreas, gallbladder, bile ducts). Gastroenterology is a subspecialty of internal medicine.