As the absence of trust between authorities hinders transnational cooperation, this doctoral research analyses how mutual trust is developed and operationalized to facilitate transnational cooperation against terrorism within the EU and ASEAN and between these regions. A data analysis matrix containing three clusters (capability building, capacity building and information sharing) is used to investigate the role of mutual trust visa-vis transnational cooperation. For this purpose, the comparative analysis between these two regions and the interregional analysis verified the applicability of the designed matrix to the hypothesis. Yet, it is not possible to draw an indisputable conclusion on the order in which one caused the other to emerge: ‘chicken and the egg dilemma'. The different elements are caught in a virtuous circle which nourishes each other. Mutual trust has thus a three-fold action: a driving force for, a support to, and a consequence of the development of cross-border cooperation.