Internal security is often hailed as a rapidly expanding area of European integration, with a growing number of strategies, policies and framework agreements in recent years. Yet actual cooperation, when viewed closely, proceeds at a halting pace – raising questions as to why cooperation appears so problematic. This book presents a novel, theoretically-informed way to understand internal security cooperation in Europe. The approach treats internal security as a "public good" requiring collective action amongst sovereign governments. All governments must contribute to the production of a public good; once produced, the public good benefits all governments. Fundamental obstacles to producing a public good thus arise, and can help explain the underlying difficulties facing European cooperation on internal security matters. The chapters in this book apply a public goods approach to different internal security issues, ranging from terrorism to border management, and from environmental security to natural disasters. Each study demonstrates how the various goals of internal security cooperation resemble different forms of public goods – and thus present different kinds of obstacles to effective cooperation. This book fills a theoretical gap in the literature on European internal security cooperation with a proven approach increasingly used in other scholarly fields.This book was published as a special issue of European Security.