Gastroesophageal reflux disease (GERD) is the most common esophageal disorder and perhaps among the most prevalent conditions seen in the primary care setting. The clinical manifestations of GERD, typical or atypical, such as noncardiac chest pain, respiratory or ear, nose, and throat symptoms, result from the reflux of gastric contents into the esophagus. Thus the clinical spectrum is wide and requires accurate diagnosis. Ambulatory 24-hr esophageal pH monitoring is not useful in all patients suspected to have GERD. This review describes the technique of ambulatory 24-hr esophageal pH monitoring, the interpretation of findings, and clinical applications of this test.