Gastroesophageal reflux disease (GERD) is a common condition with a variety of clinical manifestations and potentially serious complications. This article reviews available methods for diagnosing GERD. A clinical history of the classic symptoms of GERD, heartburn or acid regurgitation, is sensitive enough to establish the diagnosis in patients without other complications. Esophagogastroduodenoscopy is the best way to evaluate suspected complications of GERD, but endoscopic findings are insensitive for the presence of pathological reflux, and therefore they cannot reliably exclude GERD. The "gold standard" study for confirming or excluding the presence of abnormal gastroesophageal reflux is the 24-hour ambulatory esophageal pH monitoring test, and this study should be used for the evaluation of refractory symptoms and extraesophageal manifestations of GERD. A formal acid-suppression test is helpful in the evaluation of the atypical GERD symptom of noncardiac chest pain. Optimal use of currently available tests for GERD may allow for more efficient diagnosis and better characterization of the pathological manifestations associated with GERD.