In the years 1720-1820 medicine changed from an art into a science. The slow construction of clinical science, a process still to be completed, has many founding fathers. Here the transition is analyzed through the biographic studies of John Hunter (1728-1793), Domenico Cotugno (1736-1824) and Pierre Joseph Desault (1738-1795) who respectively taught in London, Naples and Paris. Because of their scientific stature they were able to plan, build and foresee the impact of these changes.