Helicobacter pylori (H. pylori) causes chronic gastritis, peptic ulcer disease, primary B-cell gastric lymphoma, and adenocarcinoma of the stomach. The overall seroprevalence of H. pylori infection in Korea was 46.6%, and the seroprevalence increased with age and was highest in patients in their 40s. Only a fraction of people infected with H. pylori develop clinical disease. Mucosal inflammation is the basic mechanism underlying the disease development in which tissue destruction may be initiated and maintained by both the bacterial toxins and immune responses by the host. H. pylori infection can be diagnosed either by invasive techniques requiring endoscopy with biopsy (histological examination, culture, and polymerase chain reaction) or noninvasive techniques (urea breath test, serology, and detection of H. pylori antigen in stool specimens). The eradication of H. pylori infection is not easy and requires combinations of antibiotics. Even with the most effective treatment regimen currently available, the eradication is not successful in about 10~20% of patients. Seven-day triple therapy (proton pump inhibitor, amoxicillin, and clarithromycin) has been the first-line therapy for H. pylori infection in Korea. In case of failure, quadruple therapy (proton pump inhibitor, a bismuth salt, metronidazole, and tetracycline) is a very effective second-line regimen. If two or more eradication treatments fail, bacterial resistance to antibiotics should be evaluated and the regimen of third-line therapy should be selected according to the antimicrobial susceptibility results.