Cardinal Walter Kasper observes that: ‘All the dialogues – with the Orthodox churches and the Anglican Communion, the Lutheran and Reformed ecclesial communities, the “Free Churches” and the new Evangelical and Pentecostal communities – converge in the fact that they centre around the concept of communio.’1 An ecclesiology of communion is the theological backstory to the 1995 encyclical Ut Unum Sint. The brainstorming that went on amongst Henri de Lubac, Hans Urs von Balthasar and Joseph Ratzinger produced a specific Communio ecclesiology. Without isolating one ‘particular version of Communion Ecclesiology as if it were the only correct version’,2 this chapter indicates the contribution which their account made to Ut Unum Sint and thus perhaps to reunion.