The fifth volume of The Oxford History of British & Irish Catholicism—covering the period from the Great War, through the Second World War and the Second Vatican Council—surveys the transformed ecclesial landscape between the papacies of Benedict XV and Pope Francis. It explores the efforts of bishops, priests and people in Ireland and Scotland, Wales and England to respond to modern challenges and reintegrate the experiences and expertise of the laity into the ministry of the Church. Alongside the twentieth century's designation as an era of technological innovation, war, peace, globalization, decolonization and liberation, this period has also been designated 'the People's Century'. Viewed through the lens of the Catholic church in Britain and Ireland, these same dynamics are explored within thematic, synoptic chapters by leading scholars. As a century characterized by the rise, or better renewal of the apostolate of the laity, this edited collection traces the struggles to reconcile tradition, re-evaluate hierarchical authority, adapt to social and educational mobility, as well as to adjudicate serious challenges from outside and within—including inflammatory biopolitics and clerical sexual abuse—to religious belief and the legitimacy of the Church as an institution.