The diagnosis of Helicobacter pylori infection represents a fundamental step for a correct clinical approach to the patient with gastritis, peptic ulcer or gastric adenocarcinoma or lymphoma. Upper gastrointestinal endoscopy has first to be made in order to obtain gastric juice and mucosal biopsies, where Hp can be found. Two different procedures are recommended to diagnose Hp infection (e.g. urease test and histology or culture). The identification of specific DNA sequences with the polymerase chain reaction represents a sensitive and specific method to diagnose Hp infection. Serum anti Hp antibodies, pepsinogen A and C determination is recommended at diagnosis and during the follow-up to assess the success of therapy.