Often invoked between Vatican II and the end of the twentieth century by both Orthodox and Catholic officials across their confessional division, the expression ""sister churches"" reflected their growing rapprochement, as well as a shift on the Catholic side from a more centralized ecclesiology to one more attentive to the local church and conciliarity. Pope John Paul II in his 1995 encyclical Ut Unum Sint spoke significantly of a ""doctrine of sister churches"" that would help guide the Catholic and Orthodox toward unity along a path of mutual respect rather than either tradition's submission to the other. In his comprehensive treatment of the history of the expression ""sister churches"" over half a century of Catholic-Orthodox relations, Dr. Will Cohen explores why the concept developed as it did, why it was so fiercely contested, and what remains vital about the concept today. In the process, Dr. Cohen illuminates the ways in which Catholic and Orthodox ecclesiology, respectively, is each most capable of renewing and sustaining its proper balance when open to the authentic gifts of the other. Will Cohen, a subdeacon in the Orthodox Church in America, is Associate Professor of Theology at the University of Scranton in Pennsylvania, and serves as President of the Orthodox Theological Society in America. He was educated at Brown University (BA, 1988), St. Vladimir's Orthodox Theological Seminary (MDiv, 2002), and The Catholic University of America (PhD, 2010).