The Reformation (alternatively named the Protestant Reformation or the European Reformation) was a major movement within Western Christianity in 16th-century Europe that posed a religious and political challenge to the Catholic Church and in particular to papal authority, arising from what were perceived to be errors, abuses, and discrepancies by the Catholic Church. The Reformation was the start of Protestantism and the split of the Western Church into Protestantism and what is now the Roman Catholic Church. It is also considered to be one of the events that signified the end of the Middle Ages and the beginning of the early modern period in Europe.

Prior to Martin Luther and the other Protestant Reformers, there were earlier reform movements within Western Christianity. Although the Reformation is usually considered to have started with the publication of the Ninety-five Theses by Martin Luther in 1517, he was not excommunicated by Pope Leo X until January 1521. The Diet of Worms of May 1521 condemned Luther and officially banned citizens of the Holy Roman Empire from defending or propagating his ideas. The spread of Gutenberg's printing press provided the means for the rapid dissemination of religious materials in the vernacular. Luther survived after being declared an outlaw due to the protection of Elector Frederick the Wise. The initial movement in Germany diversified, and other reformers such as Huldrych Zwingli and John Calvin arose. In general, the Reformers argued that salvation in Christianity was a completed status based on faith in Jesus alone and not a process that requires good works, as in the Catholic view. Key events of the period include: Diet of Worms (1521), formation of the Lutheran Duchy of Prussia (1525), English Reformation (1529 onwards), the Council of Trent (1545–63), the Peace of Augsburg (1555), the excommunication of Elizabeth I (1570), Edict of Nantes (1598) and Peace of Westphalia (1648). The Counter-Reformation, also called the Catholic Reformation or the Catholic Revival, was the period of Catholic reforms initiated in response to the Protestant Reformation. The end of the Reformation era is disputed among modern scholars.