Combined pH and multiple intraluminal impedance (pH-MII) is currently considered the best choice for GER diagnosis in preterm infants. While pH monitoring is limited to the detection of acid reflux, combined pH-MII effectively identifies both acid and non-acid GER, which is predominant in preterm babies. Moreover, thanks to its multichannel probe, combined pH-MII allows to evaluate the height reached by the refluxate within the esophageal lumen and to distinguish retrograde fluid bolus movements (reflux) from the anterograde ones (swallows).