In this chapter, students will learn about the concept and theories of international alliances, paying particular attention to the question of alliance persistence and disintegration. After discussing what alliances are, the chapter surveys the scholarly literature on why alliances form and fall apart. It then reviews the somewhat puzzling case of NATO, which many observers expected would not long outlive the Cold War. The chapter asks how well existing theories explain NATO’s persistence and concludes with theoretically informed observations about the alliance’s future prospects.