Gastric colonisation with Helicobacter pylori is common throughout the world. Although most infected individuals remain well, H. pylori is involved in the pathogenesis of type B gastritis and peptic ulcer disease. There is also an epidemiological association with gastric cancer. Person-to-person spread is the most likely form of transmission but it is not clear whether this is faecal-oral or oral-oral. The risk factors for developing disease following infection are poorly understood. The development of diagnostic techniques suited to epidemiological studies, as well as microbiological typing methods, should help to resolve these uncertainties.