This interdisciplinary Handbook provides an in-depth analysis of the complex security phenomenon of disinformation and offers a toolkit to counter such tactics.Disinformation used to propagate false, inexact or out of context information is today a frequently used tool of political manipulation and information warfare, both online and offline. This Handbook evidences a historical thread of continuing practices and modus operandi in overt state propaganda and covert information operations. Further, it attempts to unveil current methods used by propaganda actors, the inherent vulnerabilities they exploit in the fabric of democratic societies and, last but not least, to highlight current practices in countering disinformation and building resilient audiences. The Handbook is divided into six thematic sections. The first part provides a set of theoretical approaches to hostile influencing, disinformation and covert information operations. The second part looks at disinformation and propaganda in historical perspective offering case study analysis of disinformation, and the third focuses on providing understanding of the contemporary challenges posed by disinformation and hostile influencing. The fourth part examines information and communication practices used for countering disinformation and building resilience. The fifth part analyses specific regional experiences in countering and deterring disinformation, as well as international policy responses from transnational institutions and security practitioners. Finally, the sixth part offers a practical toolkit for practitioners to counter disinformation and hostile influencing. This handbook will be of much interest to students of national security, propaganda studies, media and communications studies, intelligence studies and International Relations in general.