William Cullen (d. 1790) was a key figure in the Scottish Enlightenment. Born in Hamilton, educated in Glasgow and a Professor at Glasgow University, he moved to Edinburgh in later life. Successively Professor of Chemistry, Psychology and Medicine at Edinburgh University, he was intimate friend and intellectual companion to both Adam Smith and David Hume. His innovative teaching was largely responsible for Edinburgh's growth as the foremost medical school in the 18th century world, and his numerous pupils founded medical schools and institutions around the globe, setting the pattern of medicine worldwide today. Yet, unlike Hume and Smith, William Cullen has been largely neglected in the study of the 18th century, and his contribution to the Enlightenment has until now been ignored. This new assessment aims to put Cullen and the history of science back to the forefront of Enlightenment studies.