The discovery of Helicobacter pylori is arguably the most significant advance made in gastroduodenal pathology this century. It is the most important cause of chronic gastritis, and almost certainly the major aetiological factor responsible for duodenal ulcer and probably for gastric ulcer as well. Evidence is accumulating which suggests that it may play an important role in the pathogenesis of gastric cancer. H. pylori is thought to be transmitted by the faecal-oral route or possibly oral-oral route, with iatrogenic transmission also reported. The prevalence of H. pylori infection increases with age, is commonest in developing countries, in certain ethnic minorities and those in lower socio-economic and educational groups. The organism can be eradicated using combinations of antibiotics; when treatment is successful inflammatory changes resolve, duodenal ulcers heal and do not subsequently recur.