THE INTRODUCTION: outlines Zizioulas' ecumenical career as it relates to his ecclesiology. It describes the purpose of this study and establishes the boundaries of the thesis, which aims at demonstrating that his model of communion provides a suitable basis for a Trinitarian ecclesiology. CHAPTER ONE: discusses Zizioulas' description of the way in which humans relate to each other. It examines his claim that we live in a situation of profound division and explores how he uses Trinitarian theology to suggest a way in which baptism marks an overcoming of this, leading to communion. CHAPTER TWO: examines Zizioulas' perception of Christology as constituted by Pneumatology. It suggests that this synthesis can be used as a basis for an understanding of relationships and authority in the Church and is determinative for a concept of communion. CHAPTER THREE: describes how Zizioulas views the Eucharist as an eschatological event both to construct a pattern of communion in the Church and to describe how it is realized. CHAPTER FOUR: discusses the role of the bishop in the structures of the Church. It demonstrates how Zizioulas' eucharistic understanding allows a relational model of authority to develop and explores the implications this has for ecclesial structures and patterns of authority. CHAPTER FIVE: relates the laity to the authority structures of the Church in such a way that all people may be seen to have a place within its decision processes. It analyses the extent to which Zizioulas depends on a model of communion to construct this understanding. CHAPTER SIX: describes how Zizioulas relates authority in the local Church to that of the universal Church. It examines both how the bishops exercise authority in conciliar fashion and how the whole Church receives or rejects their teaching. CHAPTER SEVEN: explores Zizioulas' use of the concept of primacy. It suggests that an ecclesiology of communion, such as that of Zizioulas, is incomplete without the concept of a universal primate. THE CONCLUSION: analyses the consistency of Zizioulas' ecclesiology and the application it can have to ecumenical debate. I argue that Zizioulas' communion-based ecclesiology is fundamentally sound, although needing some adjustments and is able to provide a stimulus to ecumenical debate.