Continuous monitoring of oesophageal pH is regarded as the gold standard for all reflux investigations. However, since gastro-oesophageal reflux is considered a normal phenomenon which occurs to a certain extent in each individual, the distinction between "normal" and "abnormal" is not always obvious and is not possible to achieve with a single test. Moreover, data depend on technical hardware such as recording devices and electrodes together with such patient characteristics as age, position, activity, and medication. Although much literature on pH monitoring has appeared recently, many investigations have failed to take into account the basic principles that are of major importance for any biomedical test such as reproducibility, sample reliability, and indications. There is a need to standardize the technique in order to compare studies performed in different centres, provided the technique is reproducible.