Johannes Cardinal Willebrands (1909-2006) certainly was one of the major players in the turn of the Roman Catholic Church to ecumenism. Through his stimulating role as secretary of the Catholic Conference for Ecumenical Questions (1952-1962), he prepared the ecumenical openness of the bishops of the Second Vatican Council. As secretary of the Secretariat for Promoting Christian Unity (1960-1968), and as President of the Secretariat, later Pontifical Council for Promoting Christian Unity (1968-1989), he played a central role in setting up ecumenical structures within the Roman Catholic Church, and starting bilateral dialogues with the churches and ecclesial communities that are not in communion with the Bishop of Rome. On the occasion of the centenary of his birthday (4 September 1909), two symposia were organised in memory of J. Willebrands, in Utrecht and in Rome.The majority of the papers presented at both conferences now become available in this volume. After a biographical introduction (A. Denaux), four contributions pay attention to 'Willebrands' formation and early commitment to ecumenism' (A. Houtepen, T. Merrigan, P. De Mey, T. Salemink). The next part focuses on 'Willebrands and the development of the Catholic view on ecumenism during the Second Vatican Council' (M. Velati, W. Thonissen, J. Wicks). Four contributions of this volume focus on 'Willebrands and ecumenism', commenting both on his relations with the Eastern Christian Churches (M. Van Parys) and with the churches of the West and to the World Council of Churches (J. Radano, W. Henn, G. Gassmann). Willebrands also played an important role in the dialogue between the Catholic Church and the Jewish people. This aspect of his work is highlighted in the next four contributions (M. Lamberigts-L. Declerck, M. Poorthuis, P.F. Fumagalli, J. Banki). The volume closes with Cardinal Kasper's address on the legacy of Cardinal Willebrands and the future of ecumenism, with some witnesses of close friends and collaborators of the cardinal (T. Stransky, P.-W. Scheele), and with a bibliography of Willebrands, provided by the co-founder of the Willebrands Archives and co-organiser of the Utrecht conference, Maria ter Steeg.