Helicobacter pylori (Hp) was discovered in 1982 by the Australians Robin Warren and Barry Marshall. Initially rejected by a skeptical scientific community, it has since gained worldwide recognition as a clinically significant bacterium. The incidence of colonization with Hp increases with age, affecting approximately one third of the world's population. Hp is uniquely capable of surviving in the acid environment of the stomach, and has properties of adherence to epithelial cells that resist parastalsis. Strains of Hp associated with human disease produce specific cytotoxic proteins. After ingestion, there is a period of intense proliferation and ensuing gastric inflammation that may result in chronic gastritis. Hp infection in children may produce symptomatic antral gastritis or duodenal ulceration. The diagnosis of Hp infection is confirmed by gastric biopsy and culture, where the organism is recognized by its characteristic histological appearance. Treatment for Hp includes combinations of bismuth amoxicillin and metronidazole administered for several weeks. In adults, chronic infection with Hp is associated with chronic gastritis, achlorhydria, and gastric cancer. An organism that was unheard of 15 years ago is now recognized as a clinically significant pathological entity. The ultimate significance of Hp as an agent of disease remains to be seen.