The chapter traces developments in ecclesiology through the twentieth century, as the ecumenical movement unfolded, and raises questions about the relationship between the church and the communion of the Persons of the Trinity, and about the nature of the Church as eucharistic and sacramental. Further more practical questions about authority, primacy, and synodality (or conciliarity) are also examined in light of the work of multilateral ecumenical dialogues (especially within the World Council of Churches), and bilateral dialogues, particularly the Anglican–Roman Catholic International Commission (ARCIC) and the international Roman Catholic–Orthodox theological dialogue. Considerable progress has been made on all these questions, but new issues have recently arisen, and these are briefly treated, including questions of imperfect communion, of the ordination of women and of those in same-sex relationships, and questions of geographical scope relative to jurisdiction and canonical territory.