Helicobacter pylori is the prototype of bacteria belonging to a new genus, the Helicobacter genus. It is a gram-negative, highly motile and microaerophilic bacterium, with a spiral shape, that colonizes the human gastric mucosa and causes several gastroduodenal diseases. Pathogenicity of H. pylori relies upon its capacity to adapt to a hostile environment and to escape the host response. Resistance to acidity, motility, adhesion, molecular mimicry, resistance to phagocytosis, synthesis of a cytotoxin, induction of an inflammatory response are the major strategies developed by H. pylori to colonize persistently and damage gastric tissue.