The gastric transitional zones are the junctional zones between the different types of mucosa: antral-body, body-cardia, and antrum-duodenum. In this article, the importance of the transitional zone in determining disease outcome, specifically duodenal ulcer, gastric ulcer, and possibly gastric cancer, is reviewed. Both gastric ulcers and duodenal ulcers are located immediately adjacent to the transitional zones. The transitional zones are dynamic rather than static areas. Local acid levels determine the behavior of Helicobacter pylori at the antral-body transitional zone and, as a consequence, the geographic distribution of gastritis in the stomach and the formation of duodenal ulcer and gastric ulcer. This review also explains that diffuse antral gastritis and multifocal atrophic gastritis are part of the same disease and not separate entities.