Long-term oesophageal pH monitoring has become widely accepted as the principle investigation for the diagnosis of gastro-oesophageal reflux pathology, especially in infants and children. Due to the number of publications applying this investigation technique, it has become necessary to standardize the various methodologies. It begins with an account of the embryology, anatomy and innervation of the upper gastrointestinal tract followed by symptoms and pathogenesis as well as an overview of the advantages and disadvantages of different investigation techniques.