This Companion provides an accessible guide for those seeking to comprehend the significance of Vatican II for Catholicism today. It offers a thorough overview of the Second Vatican Council, the most significant event in the history of Roman Catholicism since the Protestant Reformation. Almost six decades since the close of the council, its teaching remains what one pope referred to as a ‘sure compass’ for guiding today’s church. The first part of the Companion examines the historical, theological, and ecclesial contexts for comprehending the significance of the council. It also presents the key processes, as well as the participants who were central to the actual conduct of the council. The second part identifies and explores the central themes embedded in the council documents. The Companion concludes with a unique appendix intended to guide students wishing to pursue more advanced research in Vatican II studies.