Helicobacter pylori is recognized as an important cause of chronic antral gastritis and peptic ulceration. Moreover, H. pylori associated inflammatory process has been linked with gastric carcinoma. Many putative virulence factors of H. pylori have been suggested, including motility, urease and cytotoxins production and bacterial adhesins. An accessory function of CagA antigen and bacterial heat - shock proteins in the pathogenesis of H. pylori infections have been also considered. H. pylori - induced immunological response is discussed as regards local and general antibody production, the interaction of the bacteria with the phagocytes and still controversial involvement of T cells. Data on the importance of cytokines and inflammatory mediators in the disruption of the gastric mucosal barriers as well as the evidence to support a role for H. pylori as a risk factor for gastric carcinoma are also presented.