In the spring of 2005, almost by chance, the Diary of Johannes Cardinal Willebrands, which depicts events from the international ecumenical scene on the eve of the Second Vatican Council, was discovered. The Diary consists of three Notebooks that were written during the period 1958-1961. During this time, Willebrands was also appointed the Secretary of the newly established Secretariat for Christian Unity (1960). As Cardinal Augustin Bea's right-hand man, he played an important role in establishing contacts with the World Council of Churches and with other Christian churches. On the occasion of what would have been Cardinal Willebrands' onehundredth birthday (2009), his Diary has been translated and published into English, with almost one thousand critical annotations. The original Dutch text is included in the annex. The study of the diaries, in the context of the international and national ecumenical movement, can provide new insights, especially into the dialogue between the Catholic Church and the World Council of Churches in the period shortly before the Second Vatican Council. Furthermore, it provides fascinating information about the faction forming within the Vatican in preparation of the Council.