The ecumenical movement currently faces a number of notable challenges. While relations between Christian communities improved significantly over the course of the 20th century, partners in ecumenical dialogues often brought to the table fundamentally different conceptions of the nature of the church. Thus, the problem of "ec-clesiality" has emerged as an especially pressing issue for contemporary ecumenism. Fresh reflection on what makes the church legitimately the church will hold considerable promise for revitalizing efforts toward Christian unity. This book explores the contributions of the French Dominican theologian Yves Congar on the question of ecclesiality with an eye to their ongoing ecumenical potential. A pioneer of Catholic ecumenism, Congar's rich vision of ecclesiality made a substantial impact on the theological understanding of the church. From Congar's early efforts to account theologically for separated Christians to his extended reflections on the Holy Spirit after the Second Vatican Council, this critical engagement with Congar offers much-needed resources for the contemporary ecumenical situation.