Barrett's esophagus (BE) is an acquired condition in which the squamous epithelial lining of the lower esophagus is replaced by a columnar epithelium due to chronic gastroesophageal reflux. The role of acid and bile in the development of esophageal mucosal injury and the formation of BE is controversial. Acid and pepsin are unquestionably important in causing mucosal damage and BE formation in both animal models and humans. Animal studies suggest the potential for synergistic damage from conjugated bile acids and gastric acid, as well as from unconjugated bile acids and trypsin in more neutral pH settings. Evidence of the involvement of bile and its constituents in humans has been less conclusive; however, the advent of better technology to detect bile reflux is beginning to clarify the role of these constituents. Human studies show that the reflux of bile parallels acid reflux and increases with the severity of gastroesophageal reflux disease, being most marked in BE. However, recent ex vivo studies suggest that pulses of acid reflux may be more important than bile salts in the development of dysplasia or adenocarcinoma in Barrett's epithelium. Nevertheless, antireflux surgery and aggressive acid suppression with proton pump inhibitors will decrease both acid and bile refluxes, and eliminate the synergism between these two duodenogastric constituents.