The chapter explores three deeply interlinked aspects of John Zizioulas’s highly influential ecclesiology: the relationship between the church and the Trinity; the relationship between the church and the Eucharist; and finally the consequences of those relationships for the structure of the church. The church is a communion through its participation in the life of the Trinity. In Christ and by the power of the Holy Spirit, it receives and re-receives the gift of communion in every Eucharist, and communion has a shape that reflects the life of God. The Trinity is centred on the Father, and so in the church at various levels the communion of the many is centred on one who is the head. This is the purely theological reason why the synodality of the church requires primacy at the local, regional, and universal levels. The chapter concludes that, while prompting many questions and needing further development, Zizioulas’s proposal has great ecumenical value.