Functional endoscopy is a technique to analyse the upper G-I functioning under physiological conditions. Physiological conditions are those of daily life, i.e. walking, sitting, sleeping or eating, drinking and belching. Only in this way we are able to understand what happens during 24 h. This includes longitudinal observation over a 24 hr period. Observation in combination with measurement techniques like pH-metry, impedancemetry, manometry enables us to see and to measure. In this way an accurate interpretation of our results becomes possible. The aim is to get a better understanding of the physiology and pathophysiology of the upper G-I tract, particularly of digestion and reflux. The result was the identification of three different reflux-mechanisms : — Reflux-oesophagitis is caused predominantly by saliva-acid-bile-compound or saliva-acid-pancreatic-juice-compound. The local effect over time is proven. The presence of the compound is always combined with globus and burning sensation ; — The important mechanism for the extra-oesophageal complications is gas reflux (EsoPharyngeal-Reflux-Disease – EPRD). The gas model explains the great variety of the different diseases such as e.g. asthma, chronic laryngitis, laryngeal polyps and granulomas, chronic sinusitis, allergic rhinitis, recurrent otitis media and hyperplastic tonsils in infants etc. — The oesophagus acts as a filter system e.g. for the gas particles of the refluxedmaterial to prevent extra-oesophageal complications. This system is based on 3 different filtration mechanisms : the labyrinth effect, the undulation and the pressure change. This illustrates that apart from the peristaltic activity, one important mechanismof the oesophagus is the protection of the pharyngo-larynx against reflux.