The prevalence of gastoesophageal reflux disease (GERD) has been rapidly increased in Korea during last 20 years. However, there has been no systematic review regarding this disease. The aim of this article was to provide a review of available diagnostic modalities for GERD. This review includes proton pump inhibitor (PPI) test, endoscopy, ambulatory pH monitoring, impedance pH monitoring, and esophageal manometry in order to provide a basis for the currently applicable recommendations in the diagnosis of GERD in Korea. With weekly heartburn or acid regurgitation, the prevalence of GERD has been reported as 3.4% to 7.9%, indicating an increase of GERD in Korea. As the prevalence of Barrett's esophagus has been reported to be low, the screening endoscopy for Barrett's esophagus is not recommended. Several recent meta-analyses re-evaluated the value of the PPI test in patients with typical GERD symptoms and non-cardiac chest pain. That is, the PPI test has been proven to be a sensitive tool for diagnosing GERD in patients with non-cardiac chest pain and in some preliminary trials regarding extraesophageal manifestations of GERD. Ambulatory pH monitoring of the esophagus helps to confirm gastroesophageal reflux in patients with persistent symptoms (both typical and atypical) in the absence of esophageal mucosal damage, especially when a trial of acid suppression has failed. Impedance pH test is useful in refractory reflux patients with primary complaints of typical GERD symptoms, but this value has not been proved in patients with non-cardiac chest pain or extraesophageal symptoms. This systematic review is targeted to establish the strategy of GERD diagnosis, which is essential for the current clinical practice.