Tube-based ambulatory pH testing has rapidly evolved in the past 3 decades to become the standard by which other diagnostic approaches to gastroesophageal reflux disease are judged. Acid exposure 5 cm above the manometrically determined lower esophageal sphincter is the standard for documentation of pathologic acid exposure of the distal esophagus. Proximal esophageal or hypopharyngeal monitoring is an evolving technique that may shed light on patients with supraesophageal symptoms. The ability to simultaneously monitor esophageal and gastric acidity (usually in patients with persistent symptoms despite therapy) is another advantage of this technique. Whether the new system that allows simultaneous pH and impedance monitoring and the system that uses an implantable tubeless monitoring capsule will supplant this older but well-established technology remains to be determined.