In this review article we examine several facets of extended pH monitoring of the esophagus. The different types of instrumentation and the technical difficulties of obtaining and interpreting tests are discussed. The wide variations among different laboratories in the duration and timing of pH monitoring sessions are summarized. We outline the problems in interpretation and reproducibility such as normal control values, criteria for separation of abnormal from normal subjects, correlation of pH events with symptoms and with esophageal injury. Comparison of extended with short-term monitoring of esophageal pH are drawn, as well as comparisons between results of extended monitoring and other tests of reflux. Cost considerations are listed. Finally, we present a proposed scheme for the use of extended pH monitoring as well as a cautious note about accepting extended pH monitoring as a routine clinical test at present.