This edited collection offers a comprehensive, multidisciplinary examination of how the EU has addressed violent radicalization as a key precursor to terrorism. Set against the backdrop of the post-9/11 security landscape and the evolving threats of jihadist, far-right, online, and lone-actor extremism, the volume analyzes the gradual shift from state-centered counterterrorism to preventive and cooperative frameworks. The book combines international, legal, and policy analyses with local and individual-level perspectives. The first part explores EU-level policies, legal competences, and cooperation mechanisms developed to prevent violent radicalization, highlighting both achievements and structural limitations. Chapters examine the evolution of EU prevention strategies, the adequacy of EU law to address emerging trends, the development of the European Security Union, and the risks of counterproductive securitization through concepts such as the “extremism trap.” The second part focuses on emerging threats and practical prevention experiences, emphasizing gaps in existing responses. It addresses underexplored phenomena, such as incel ideology, far-right digital activism, and lone-actor terrorism, as well as the role of mental health, education, and identity dynamics in radicalization processes. The chapters in this second part include empirical studies and program evaluations largely drawn from European and Spanish contexts. This book will be of interest to academics, researchers, and advanced students in the fields of terrorism and counterterrorism studies, radicalization, the prevention of violent extremism, international law, European politics, criminology, and security studies more broadly.