There is now substantial evidence that Campylobacter pylori (Cp) is able to colonize the gastroduodenal mucosa and is responsible for active chronic gastritis, its role in duodenitis, gastric ulceration and duodenal ulceration is still under debate. Cp has a lot of characteristics which are prerequisites for a pathogen: the typical S-shape, the corkscrew-like movement and the powerful urease and protease enzymes. These features allow a rapid movement through the mucous layer to permit access to the apical membranes of the surface mucous cells. There they adhere directly to the membranes and induce several ultrastructural alterations: degeneration of microvilli, depletion of mucous granules and an increase in sialic-acid rich glycoproteins in the apical part of the cytoplasma. Cp weakens the tight-junction complex and is found between the cells and sometimes intracellularly. Cp is phagocytized by invading polymorphonuclear leukocytes and causes an intense inflammatory response. These observations clearly demonstrate pathological alterations which in the cellular level induced by Cp with the result of a disrupted mucosal barrier of the stomach and the duodenum.