The article studies the implementation of the Catholic Reformation in Portugal under Spanish kings Felipe II, Felipe III, and Felipe IV between 1580 and 1640, when the country's monarchy was subsumed under the Spanish crown. It examines the roles of, and relationships between, different interests and powers in this process, including the monarchy, the papacy, bishoprics, and local authorities. The article maintains that, despite difficulties encountered in introducing reforms mandated by the Council of Trent, the reform of the Portuguese monarchy was completed successfully by the mid-17th century.