Comparing countries' strengths and military capacities is a big part of global security. It affects power balances, arms races and arms control. Status hierarchies come from comparing states. Fixing security issues like fragile states, maritime security and cyber threats is guided by comparisons of who's most affected and how bad it's getting. Even though comparing is important, research rarely focuses on it. Instead, it studies related things or compares things separately. This causes separate research streams, all studying how comparisons affect security but not talking to each other. This book aims to unite these streams, discussing how comparisons shape global security and asking three questions: How are comparisons made? How do they matter politically? How do they shape security?