Although the Counter-Reformation is sometimes also called the Catholic Reformation, the latter term properly refers to the set of measures of spiritual, theological and liturgical renewal with which the Catholic Church had attempted to reform its institutions even before the Council of Trent. During the Council of Constance, for example, the council fathers had already called for a reform "in the head and in the members", but it was only after the Protestant Reformation that this need became urgent, resulting in the application of the Tridentine conciliar provisions.

The Counter-Reformation was the Catholic Church's specific reaction to the Protestant Reformation, and it is often characterised as the acceptance of certain demands for ecclesiastical renewal, which Luther, Calvin, and other reformers had also emphasized. The Catholic Church began to change various aspects such as the training of priests, residence requirements for bishops, and combating immorality among the clergy, but the Counter-Reformation was also quite polemical and defensive to prevent other Catholics from converting to the Evangelical Churches. Some forms of these defensive measures consisted of catechisms, sacred art, and popular devotion, but the Church also resorted to violent repression of Evangelical Christianity, with the Vatican often collaborating with Catholic countries on the continent.

Thus, historians normally identify the period from the opening of the Council of Trent to the Peace of Westphalia, which ended the Thirty Years' War, as the "Age of the Counter-Reformation," and by then, Europe had already suffered through one of the deadliest wars in history.