Book excerpt: The region of South East Europe (SEE), which is home to both NATO and Partnership for Peace (PfP) countries, serves as an important corridor between Europe and the Middle East, North Africa, and the Caucasus. In recent years, however, SEE has also experienced high levels of cross-border, military and defense-related challenges in the form of migration, smuggling, terrorism, and cyber threats. Furthermore, the use of the new information environment (IE) to further extremism in SEE and elsewhere in NATO and PfP countries has had far-reaching command and control (C2) implications for the Alliance. A collaborative interdisciplinary, international and regional approach is clearly needed to adequately assess and address these hybrid threats. This book presents papers delivered at the NATO Science for Peace and Security (SPS) event: “Senior Leadership Roundtable on Military and Defense Aspects of Border Security in South East Europe”, held in Berovo, the Former Yugoslav Republic of Macedonia* from 23-30 September 2017. The aim of this special SPS grant was to maximize opportunities for extensive dialogue and collaboration between senior regional members, and the almost 70 distinguished academic and legal experts, as well as current or former senior-level practitioners from various governments, NATO bodies, and international organization that participated. It was the first SPS event of its kind in SEE as well as the first NATO SPS grant to be co-executed by the U.S. Department of Defense via the U.S. National Defense University. Other co-organizers were the C4I and Cyber Center of Excellence at George Mason University and PfP partner institution, the General Mihailo Apostolski Military Academy – Skopje, Associate Member of the University of Goce Delčev – Stip. The book is divided into five parts: global trends, defining the problem, policy and academic solutions, national and regional case studies, and technological solutions. It will prove an invaluable source of reference for all those with an interest in the SEE region as well as cross-border hybrid threats, in general. * Turkey recognizes the Republic of Macedonia with its constitutional name.