For more than two centuries in which Catholicism was illegal in Scotland, the Scots Colleges abroad operated as a sixth Scottish university. During this time the university's alumni, individually and collectively, helped to ensure the survival of Catholicism in Scotland through political and military activity as well as missionary work. Earlier scholarship has treated the colleges individually and overlooked the degree to which the university corpus formed coherent networks which, over two centuries, made significant contributions to greater European cultural and intellectual movements. Through a number of examples, a picture is given of the hitherto little recognised Scottish Catholic contribution to developments in the Arts, Humanities and Sciences in the seventeenth and eighteenth centuries.