Inhibition of the proton pump in the parietal cells has been established as the main therapeutic principle in the treatment of acid-related diseases, such as peptic ulcer and gastro-oesophageal reflux. The proton pump inhi bitors are tailored for their purpose. They accumulate in the target cell, are activated by acid and bind strongly to the specific target - the proton pump. The clinical superiority of the proton pump inhibitors is due not only to their high efficacy but also to the long duration of the acid inhibition in comparison with other antisecretory drugs. At present when drug discovery mostly relies on identification and characterization of potential targets by genome research, molecular biology, combinatorial chemistry and automated screening, it seems worthwhile to present the development of the tITst proton pump inhibitor - omeprazol- starting from a chemical structure with an observed antisecretory effect but also severe toxic effects that had to be eliminated. As always, basic and applied research operate luind in hand to optimize the delicate balance be tween efficacy and safety of a new drug. This goal often involves time and many different specialists.