This book provides a detailed theoretical and practical analysis of a key concept in cooperation, namely mutual trust, in the EU and ASEAN regions. It particularly looks at the issue of counterterrorism. It analyses the differences in the legalistic approach of the EU and the consultation and consensus-based approach in the ASEAN region, and the correlation between mutual trust and regional and interregional cooperation against terrorism. Thereby, this book extends the well-known concept of counterterrorism in EU criminal law to another regional case in order to analyse its possible use and operationalisation in other regions. It provides important findings on whether and, if so, how the EU and ASEAN legal frameworks have facilitated intra-regional – and potentially interregional – cooperation. In order to measure trust, it focuses on information sharing, and discusses the terminology related to this matter, the mechanisms of cooperation, and the difficulties and successes of information sharing. This book is of great interest for academics doing research on trust, transnational cooperation, regional counterterrorism legal architecture in the EU and ASEAN, and comparative legal regional and interregional analysis, and officials and practitioners involved in transnational cooperation in criminal matters, especially terrorism.