The introduction to this book defines the gun gap, which refers to differences in political behavior and attitudes between gun owners and nonowners. In addition, the introduction establishes why the gun gap is important for understanding modern mass politics. Election analysts and scholars typically overlook the gun gap, but it is an essential explanation for an impressive range of political behaviors and attitudes, including voter choice and turnout, perceptions of personal and public safety, preferences for gun control policies, and support for the death penalty and other punitive measures. Finally, the introduction outlines the book chapters, discussing key theories and findings.