In the last three decades, esophageal pH monitoring has progressed from an exclusively laboratory-based physiological research tool to a routine outpatient clinical investigation in patients with one of the most common gastrointestinal disorders. Technological advancement has considerably simplified both the procedure and the interpretation of data obtained, and there is currently reasonable consensus as to the parameters that best discriminate between physiological and pathological gastroesophageal reflux. There remains a need for internationally agreed definitions and standards, particularly with regard to calculation of these parameters, which would improve comparability of future research studies from different centers. Clinical applications of the technique have increased with our enhanced knowledge of the protean clinical manifestations of reflux disease. The advent of potent pharmacotherapy and the recognition of a need for ongoing treatment in many patients will ensure that the most sensitive test for the diagnosis of gastroesophageal reflux disease will remain an essential part of the investigation of upper gastrointestinal symptoms.