Gastric acid produced by the parietal cells has a significant role in a variety of gastrointestinal diseases, such as duodenal ulcer, gastric ulcer, and gastroesophageal reflux disease. We address acid production by the parietal cell and the pharmacological means to control gastric acid secretion. The physiology of parietal cell acid secretion is reviewed. The role of acid-suppressive therapy with histamine H2 antagonists and omeprazole in duodenal ulcer, gastric ulcer, and gastroesophageal reflux disease is discussed. Pharmacology, side-effect profile, and dosage requirements of the histamine H2 antagonists and omeprazole are outlined.