This edited volume offers a detailed analysis of Finland's transformation from neutrality to a pivotal NATO member following Russia's invasion of Ukraine in 2022. With contributions from scholars and military practitioners, the volume fills a critical gap in the literature by focusing on Finland's changing security landscape. It explores Finland's emerging role within NATO, changes in Finnish strategic posture, the perspectives of key allies, and its evolving relations with Russia and China. Divided into three thematic sections, the book covers Finland's shifting military-strategic context, its changing security orientation with great powers, and the impact on its key national security practices such as conscription, total defence, and border security. Finland's geostrategic importance is emphasised, particularly as NATO's frontline state with the longest border with Russia, and as a key player in Arctic policy. In contrast to broader regional analyses, this book provides a comprehensive and focused examination of Finland's national security in its own right, offering vital insights for scholars and policymakers. This book will be of much interest to students of Strategic and Security studies, NATO, Nordic studies and International Relations. The Open Access version of this book, available at http://www.taylorfrancis.com, has been made available under a Creative Commons Attribution-Non Commercial-No Derivatives (CC-BY-NC-ND) 4.0 license.