The Second Vatican Council (1962-1965), or Vatican II, is arguably the most significant event in the life of the Catholic Church since the Reformation. The Council initiated, intentionally or not, profound changes not simply within Catholic theology, but in the religious, social, and moral lives of the world's billion Catholics. It also reconfigured, intellectually and practically, the Church's engagements with those outside of it-most obviously with regard to other religions. The sixteen documents formally issued by Vatican II constitute some of the most influential writings of the whole twentieth century. Debates over their correct interpretation and authority are constant, but they remain an indispensable point-of-reference for all areas of Catholic life, from liturgy and sacraments to the Church's vast network of charitable and educational endeavors the world over. In this Very Short Introduction, Shaun Blanchard and Stephen Bullivant present the backstory to this event. Vatican II is explored in light of the wider history of the Catholic Church and placed in the tumultuous context of the 1960s. It distils the research on Vatican II, employing the first-hand accounts of participants and observers, and the official proceedings of the Council to paint a rich picture of one of the most important events of the last century.