This is the first book-length analysis of the term unity as used within recent ecumenical discussion. It seeks to present in a clear and useful way the many conceptions of unity found in discussion, whose subtle diversity and number are often a source of confusion and ambiguity. It is intended primarily for ecumenical participants and students; but the methodology is of wider interest because it employs taxonomy (of a kind usually found within biological sciences) to analyse and present conceptual diversity.