Our stomach produces daily a large amount of hydrochloric acid at a pH close to 1. This secretion represents a more than one million-fold in H+ ion concentration as compared to blood and intracellular medium of a pH of 7.4. It is due to a specialized cell, the parietal cell, which contains a unique transport enzyme, the (H+, K+)-ATPase. The parietal cell is itself under the control of several endocrine and paracrine stimuli operating through appropriate receptors. The recent progresses achieved in the knowledge of these key elements of the acid secretion mechanism had led to the development of specific and potent inhibitors of crucial interest in the treatment of ulcer disease and reflux oesophagitis such as histamine H2 and (H+, K+)-ATPase antagonists.