Over the years there have been many treatments of Paul's theology that have focused on what the churches he wrote to were like, and what that might mean for today. However, what Paul says about relationships between churches has been frequently neglected, or only briefly considered. This book analyzes Paul's use of the word "church" as well as family imagery, holiness language, body imagery, and Paul's understanding of imitation and apostleship to demonstrate the breadth of his understanding of relationships between churches, of inter-church solidarity. Inter-church solidarity is shown to be integral to Paul's understanding of church from the earliest letters, and the book exposes a rich tapestry of relationships that should challenge and encourage the church in the twenty-first century.