Behind every major policy decision lies an idea, a debate, and a network of minds shaping the course of governance. This encyclopedia offers a comprehensive and accessible journey into the world of European and North American think tanks–institutions that stand at the crossroads of knowledge and power. By mapping their structures, intellectual traditions, and policy influence across regions and sectors, the book reveals how research moves from paper to practice, from analysis to action. Bridging scholarship and policymaking, it serves as both a reference work and a reflection on the quiet yet profound role of ideas in shaping contemporary political, economic, and security agendas.