H. pylori infection is strongly implicated in the pathogenesis of peptic ulcer disease and gastric cancer. Although the infection may be detected in over half the world's population, only a proportion will develop either ulcer disease or gastric cancer. The prevalence of the infection is very high in both diseases, but other factors combined with chronic infection must lead to specific clinical manifestations of the disease. Long-standing infection results in chronic active gastritis. In a certain subset of the H. pylori-infected population, factors such as dietary, environmental, and/or genetic factors, or specific strains of the bacterium, may result in the expression of one disease or another. There are now many investigations into these various aspects of association of the infection and disease outcome which may lead to elucidation of the actual processes in the development of specific disease in the H. pylori-infected patient. Many different tests are now available for accurate diagnosis of the infection. The non-invasive tests include the serologic tests, which include both the ELISA and the rapid-immunoassay, and the urea breath test. Invasive tests can be performed on the patient at the time of endoscopy when tissue specimens can be taken for the rapid urease test, histology, or culture.