Reformation, the religious movement that established Protestantism as a major division of Christianity. The Reformation arose in protest against many Roman Catholic doctrines and practices. In the beginning the leaders tried to reform from within, but later they broke away from the church, and most of the countries of northern Europe became Protestant. The Reformation began with Martin Luther's challenge in 1517 and continued for most of the 16th century. After 1545 there developed a Catholic Reformation, commonly called the Counter Reformation. The period of the Reformations included a series of wars that ended in 1648.

Although the Reformation seemed primarily a religious upheaval, it was also a complex social and political movement. A revolt against the religion and way of life of the Middle Ages, it was part of the transition from medieval to modern times. The revolt accompanied a change in the political, economic, intellectual, and ecclesiastical traditions of the medieval age of faith. The results touched every aspect of human life.