It is well known that Helicobacter pylori is able to colonize the gastric mucosa, causing a chronic and persistent infection with complications, such as peptic ulcer and gastric cancer. This review places emphasis on some epidemiological aspects of Helicobacter pylori infection and its mode of transmission. At the same time, invasive and non-invasive methods of diagnosis of Helicobacter pylori infection are illustrated. More space is devoted to the host response following invasion of the stomach. In this respect, the role played by different growth factors and polyamines in the course of Helicobacter pylori disease is discussed also in relation to the result of eradicating treatment. On the other hand, an accurate description of the host immune responses against Helicobacter pylori organism and/or their components (e.g. lipopolysaccharides) is reported. Finally, since Helicobacter pylori has been classified as a class I carcinogen, current researches are focussed on the Helicobacter pylori-induced carcinogenesis.