NATO is the perennial survivor. Crises come and go, yet still it persists. Two decades since the watershed of the Cold War, this book investigates NATO's staying power. From the collapse of the Soviet Union through wars in the Balkans, and interventions in Afghanistan and Libya, the Alliance has adapted and managed to attend to new roles and purposes. What explains NATO's longevity and what can we infer from its recent past in assessing its possible future? And what has been the nature of its development - is this driven by the interests of its powerful members, by institutional flexibility or the glue of a set of common values? This book tackles such questions. It investigates the transformation of the Alliance through the lens of International Relations theory and a set of case studies that look at operations, enlargement and relations with Russia and the EU. NATO, it concludes, will continue, but it will remain subject to ongoing crises and challenges of change.