The University of Leyden was founded in 1575 as the reward of the city's endurance of the Spanish siege in 1574. Its influence on botany in the seventeenth and eighteenth centuries is part of its far-reaching influence during this period on medicine, to which botany was then ancillary. In this it was the successor of Montpellier and Padua. The first university founded after the Reformation to practise and maintain religious tolerance towards its students, Leyden became the great international university of Europe, drawing students from Scandinavia, Germany, Switzerland and France, from all parts of the British Isles and the British American colonies (roughly 4,000 English-speaking students between 1600 and 1750) and even from Barbados, Jamaica and Constantinople. It offered facilities for higher education then denied, for example, to dissenters in England or else not available, as in Scandinavia. Owing to this religious tolerance in an age of intolerance and also to the personal eminence of a succession of professors, its influence spread widely. Directly and indirectly, Leyden made its greatest contribution to botany and medicine through the work and personality of Herman Boerhaave (1668–1738) and led to the founding or restoration of botanic gardens at Edinburgh, Göttingen, Uppsala and Vienna. Beginning with Clusius, its influence upon botany may be traced through Hermann and Boerhaave to Haller, Linnaeus, Lettsom and others. No other university has a more sustained and continuous record of service to botany and medicine during these two centuries than Leyden. This paper also touches upon the history of other universities.