The Reformation in Denmark–Norway and Holstein was the transition from Catholicism to Lutheranism in the realms ruled by the Danish-based House of Oldenburg in the first half of the sixteenth century. After the break-up of the Kalmar Union in 1521/1523, these realms included the kingdoms of Denmark (with the former east Danish provinces in Skåneland) and Norway (with Iceland, Greenland and the Faroe Islands) and the Duchies of Schleswig (a Danish fief) and Holstein (a German fief), whereby Denmark also extended over today's Gotland (now part of Sweden) and Øsel in Estonia.