The period between the publication of the encyclical ‘Divini Red-emptoris’ (On Atheistic Communism) in 1937 and the election of the Polish Pope, John Paul II, was marked by a definite and fundamental change in Vatican/Communist relations. This study attempts to chart this development, explaining exactly how and why this transformation has occurred. The few other studies of this subject have all lacked a historical perspective, and have therefore tended to encourage the false notion that the views and action of the successive popes have remained the same. Part I of this essay examines the hostile intransigence of Pius XI and Pius XII during the Second World War and the Cold War that followed, and centres upon the startling change of attitude promoted by Pope John XXIII and the Second Vatican Council. The more cautious but perhaps more significant promotion and extension of this Joh-anine ideal by Paul VI over a longer period of time will be analysed (along with a brief look at the state of Vatican/Communist relations in the present day). In Part II this study is essentially Vatican centred; the Church is large and often ambiguous, and local hierarchies, groups of militant lay Catholics, Christian Marxists, prominent theologians, and individual clergy, have only been analysed when they influence or have helped to change the Holy See. Papal speeches and letters, promulgations and edicts from the Holy Office and the other Vatican Congregations, and particularly Papal encyclicals, have been used extensively, and other primary material such as journals, reviews, newspapers and other contemporary writings have been consulted where necessary. The change in the nature of communism and the policies of the communist world (primarily the Soviet Union and the Eastern bloc, but also China, Indo-China, Latin America and the European Communist parties) I have treated as a secondary development due to limitations of time, space and sources.