While twentieth-century Welsh Nonconformity and the Anglican Church in Wales have both received historical attention, there is no similar treatment of the Roman Catholic Church. This study redresses this imbalance, and is based largely on previously unseen evidence. The book begins by charting and accounting for the remarkable growth of the Roman Catholic Church in Wales between the formation of its Province of Wales in 1916 and the commencement of the Second Vatican Council in 1962. This growth was at a time when Nonconformity, hitherto predominant in Welsh religion, began its spectacular decline. The book goes on to examine critically the reaction to the Catholic expansion, both from within and outside the Church. This new paperback edition of the book includes a fully updated bibliography and a new chapter exploring progress in the field since the book’s original publication.