The incidence of gastroesophageal reflux disease (GERD) is increasing and if left untreated can lead to significant patient morbidity and even death. The disease results from the abnormal reflux of gastric contents into the distal esophagus causing symptoms in most and subsequent mucosal damage in some. Several investigations can be used to confirm the diagnosis, but most are dependent on the presence of sequelae and complications of the disease. The physiologic test of ambulatory 24-hour esophageal pH monitoring has proved to be the most sensitive and specific diagnostic investigation. It measures increased esophageal exposure to gastric juice by detecting the concentration of hydrogen ions (pH <4) in the distal esophagus. The technique measures gastric juice exposure at a point 5 cm above the manometrically determined upper border of the lower esophageal sphincter. The exposure is measured in components of frequency of reflux episodes, duration of reflux episodes, and accumulated exposure time. The components are integrated into a composite score, which is reproducible, gender and race independent, and correlates with the degree of esophageal epithelial damage determined histologically. The composite score has been shown to be the most reliable measurement of a therapeutic acid suppression regimen or an effective antireflux operation.