Gastro-esophageal reflux disease (GERD) refers a condition that develops when the reflux of stomach contents causes troublesome symptoms and/or complications. The disease was subclassified into esophageal and extraesophageal syndromes in the new Montreal Definition. Hiatal hernia, decreased lower esophageal sphincter, transient lower esophageal sphincter relaxation, esophageal acid clearance, and delayed gastric emptying might be implicated as the pathogenesis of esophageal syndrome. Although non-erosive reflux disease (NERD) is included in the esophageal syndrome, it might be different from reflux esophagitis because of the lower response rates to acid suppression with proton pump inhibitors. Esophageal visceral hypersensitivity, sustained esophageal contractions, and abnormal tissue resistance are thought to be the mechanisms of NERD. Further investigations for the pathogenesis of each classification are expected.