Like the Irishman giving directions, questions about the exercise of authority in the Church tend to elicit the answer, ‘I wouldn't start from here.’ In this useful study, Paul Avis asks whether the conciliar movement in the west, which reached its zenith with the Council of Constance (1414–18), might help the cause of Christian unity in the twenty-first century. Avis provides a detailed narrative of the historical background, and guides the reader with clarity through the intricacies of the Great Schism, in which two, and then three, popes were regarded simultaneously by competing factions as the lawful successor of St Peter. Subsequent sections of this ambitious book can feel somewhat rushed and lacking in fluency: better editing would have improved the finished text. (Another editorial niggle is that the inclusion of references within the main text becomes tiresome: one yearns for footnotes or endnotes instead.) Nevertheless, the breadth of Avis's horizon does ensure that the reader is able to put the issues in the widest context, and to compare ecclesiological models across centuries as well as traditions. Avis writes well on Vatican I and papal primacy, and the way in which this is nuanced by Vatican II. He refutes the caricature understanding of infallibility, while demonstrating just how difficult it is for the Roman Catholic Church to show precisely what it does believe about the relationship between the authority of the supreme pastor, the bishop of Rome, and the entire college of bishops. When Avis writes on natural law, one feels that he is less confident, and some of his assumptions are questionable. (He acknowledges a range of views, but still inclines to treat Aquinas's use of natural law too much in isolation from the rest of his theology). If Avis's exploration of the theme of conciliarism is comprehensive, the answer to his original question – whether, as a model, it now helps – must be at best a highly qualified ‘yes’. What of the present Anglican context? Avis suggests that Anglicans have come closer than anyone to the principles of conciliarism. Yet, as he concedes, the Anglican Communion showed itself unprepared for the ordination to the episcopate of Gene Robinson, ‘a brutal and unavoidable new fact in the Anglican ecclesiastical landscape’, where ordination itself is held to be ‘an ecclesial sign with universal intention’. The benefits of a ‘bishops-in-synod’ model of church government, which is not dissimilar from conciliar ideals, are often cancelled out by the adversarial and quasi-parliamentary procedures into which the model is shoe-horned. Finally, while Anglicans want to assert that ‘each local church is fully the Church of Christ … the truth is that the unity of the Church demands a visible, physical symbol and Anglicans increasingly recognise this’. Somehow, it seems that the Church needs Peter. ‘The right combination of conciliarity, collegiality and primacy is the Holy Grail of modern ecumenical dialogue’, writes Avis (p. 184). Whether medieval conciliarism provides the answer, Avis has written a book which will certainly help to take that dialogue forward.