A very sizeable research project concerning Calvinism in Friesland, roughly between 1550 and 1650, has just been concluded. Based in part on an analysis of 350 parishes, later combined into Reformed parishes, the project was aimed at creating a picture of the ‘Calvinisation' of Friesland. The central issue is the number of Reformed and other religious practitioners and the way in which people of different faiths co-existed. The conclusion is that a so-called ‘interactive ecumenicity' arose.