Gastroesophageal reflux disease (GERD) is diagnosed according to the medical history or in response to proton pump inhibitor therapy. However, the need for further testing is always appropriate. The decisive evidence for the current diagnosis of GERD is severe erosive esophagitis of Los Angeles grade C or D, long-segment Barrett’s mucosa or peptic strictures seen on endoscopy or an acid exposure time >6% on ambulatory pH or pH impedance monitoring. If ambulatory reflux monitoring correlates between reflux and the symptoms, then the diagnosis and treatment are certain. If it is difficult to clearly diagnose this malady as seen upon endoscopy and ph/pH impedance monitoring, then this review recommends the biopsy findings, motor evaluation and novel impedance metrics. Novel impedance metrics include the baseline impedance and the post reflux swallow-induced peristaltic wave index. Therefore, making a future GERD diagnosis should focus on defining the patient's phenotype. The phenotype is determined by the level of reflux exposure, clearance efficacy, anatomy of the esophageal gastric junction, and the psychological state of the patient. The purpose of this review is to clarify the diagnostic guideline for GERD according to several test methods. (Korean J Gastroenterol 2019;74:321-325)