Disinformation has recently become a salient issue, not just for researchers but for the media, politicians, and the general public as well. Changing circumstances are a challenge for system and societal resilience; disinformation is also a challenge for governments, civil society, and individuals. Thus, this book focuses on the post-truth era and the online environment, which has changed both the ways and forms in which disinformation is presented and spread. The volume is dedicated to the complex processes of understanding the mechanisms and effects of online propaganda and disinformation, its detection and reactions to it in the European context. It focuses on questions and dilemmas from political science, security studies, IT, and law disciplines with the aim to protect society and build resilience against online propaganda and disinformation in the post-truth era.