Helicobacter pylori plays an essential role in the development of several both acid-related and neoplastic gastroduodenal pathologies. There are still uncertainties about the transmission routes and the sources of H. pylori infection. Man is the only well established "reservoir" of H. pylori, while the role of other mammalians (cat, pig, primates), as sources of infection, is still controversy. Literature data suggest four different modalities of transmission of the infection: faeco-oral, oro-oral, gastro-oral, gastro-gastric. By faeco-oral route, the bacterium, excreted with faeces, might colonize water sources, becoming so available to be transmitted to man and other mammalians. By oro-oral route, H. pylori, which colonizes dental plaque and saliva, may be transmitted by saliva to other individuals. The gastro-oral route is the typical modality of transmission in the childhood, when H. pylori uses the mucous achlorhydric vomitus of the children to infect a new host. Finally, by gastro-gastric route the bacterium might be transmitted by endoscopic procedures. In conclusion, we believe the different modalities of transmission may be contemporaneously involved, since none per se is able to explain the widespread occurrence of H. pylori infection.