The Second Vatican Council (Latin: Concilium Oecumenicum Vaticanum Secundum or informally known as Vatican II) addressed relations between the Roman Catholic Church and the modern world. It was the twenty-first Ecumenical Council of the Catholic Church and the second to be held at Saint Peter's Basilica in the Vatican. The council, through the Holy See, formally opened under the pontificate of Pope John XXIII on 11 October 1962 and closed under Pope Paul VI on 8 December 1965.

Of those who took part in the council's opening session, four have become pontiffs to date: Cardinal Giovanni Battista Montini, who on succeeding Pope John XXIII took the name of Paul VI; Bishop Albino Luciani, the future Pope John Paul I; Bishop Karol Wojtyła, who became Pope John Paul II; and Father Joseph Ratzinger, present as a theological consultant, who became Pope Benedict XVI.

Currently, the questioned validity of the Second Vatican Council continues to be a contending point for religious communities who are not in full communion with the Roman Catholic Church. In particular, two schools of thought may be discerned: