Prolonged esophageal pH-metry has become an irreplaceable investigative tool in diagnosing gastroesophageal reflux. Given the presence of episodes of acid reflux even under physiological circumstances, it is of primary importance to define the range of normal values in order to correctly identify the condition of disease. This task is complicated by the lack of an absolute and objective marker of normality, to be referred to, and by the number of variables that can influence the measurement of the esophageal pH. Among the most important factors that can influence this test there are age, physical exercise, posture and diet. Even voluptuary habits, such as smoking and alcohol intake, could be detrimental in this respect. Then there are many technical factors that can act as sources of relevant errors in measuring and calculating the parameters chosen to discriminate normality from abnormality, the most important ones being as follows: defective electrodes, electrodes that have not been calibrated accurately, electrodes positioned erroneously, interference with the pH readings by mucus, food, aiming at the walls of the esophagus, inconsistent sampling rates among the recording equipment, differences in the software used to automatically analyze the experimental data, failure to recognize and consequently to eliminate the artifacts present in the pH-metry tracings, inadequate "in situ" repeatability of the esophageal pH measurements on the part of paired electrodes that simultaneously record the pH.