Résumé : As transnational crime has no regard to borders, competent authorities need to overcome the barriers the barriers of national jurisdictions and cooperate together. The UN and regional organisations, including the EU and ASEAN, have required States to criminalise transnational crime, including terrorism, and to implement effective measures to prevent, investigate, detect, prosecute and punish these offences. They have also required States to cooperate; cooperation being one of the key measures to ensure the effectiveness of States’ action.The regional level is considered to be the most effective to harmonise legislation as well as to adopt and implement mechanisms of cooperation for security purposes. However, the nature and level of cooperation vary a lot depending on the regional framework considered. In the EU, this cooperation has been legally framed and institutionalised as well as associated with harmonisation of Member States’ legislation. By contrast, cooperation remains much more informal in ASEAN and is not associated with harmonization of legislation. Despite its low level of integration, ASEAN plays an increasing role in the region by leading efforts to create a regional legal architecture. ASEAN is the most successful regional grouping in the “developing world” and has a particular approach vis-à-vis terrorism. Both regions have therefore a legitimate ground and purpose in learning from each other (comparative regionalism) and in working together (interregionalism) in order to ensure – up to the capacities of each regions – the best response to terrorism.The differences between the two regions are due to various factors, among which the unalike degree of trust between their respective Member States and their capacity to cooperate. Despite its absence in the constitutional treaties, “mutual trust” is a concept constantly used in the AFSJ, especially when police and judicial cooperation is at stake. Whereas its precise status, nature and consequences is still debated, mutual trust seems to have reached an institutional level and to have been transformed into legal expectations in the EU. By contrast, although trust is repeated in ASEAN declarations and objectives, it is less visible in practice between ASEAN Member States. Mutual trust is a fundamental yardstick in developing cooperation mechanisms. The highest the confidence in each other’s systems is, the more efficient the cooperation. Mutual trust serves to build bridges between national jurisdictions. In the EU, such mutual trust is based on common values and norms and implies the development of common minimum standards in the field including human rights norms. It rests of course also on other factors such as mutual understanding of the threat and each other’s particular legislation.The compared regional analysis will highlight notable discrepancies in each region’s approach. The EU has gone through a tremendous institutional evolution by communitarising the AFSJ. The EU has adopted norms to facilitate cooperation based on common standards and mutual trust. With the numerous types of cooperation mechanisms, the EU adopted a two-fold objective, namely facilitating the cross-border cooperation and harmonising the HR standards. By contrast, ASEAN is still governed by the so-called ‘ASEAN Way’ based on consultation and consensus rather than on bargaining and give-and-take leading to deals enforceable in a court of law; on non-institutionalised processes; and on practice-based rules. These differences involve a clear lack of trust between Member States. This method is slowly and partially shifting towards a more institutional and rules-based approach. This comparative analysis will allow to conclude on the convergences and differences in the conditions and level of trust in each of the two regions in the field of terrorism and on their impact on the level of cooperation.Based on this comparison, interregional challenges and prospects will be focused on. The EU has adopted a comprehensive approach combining human rights and security measures. Preserving such a balance when interacting with external actors is a serious challenge, especially when States tend to privilege security measures. The EU must adapt its objectives, priorities and means to the particularities of ASEAN while respecting its own standards. In this regard, mutual trust is an important factor in developing interregional collaboration. Since 1972, the EU developed economic ties with different ASEAN Member States and with the ASEAN Secretariat itself. Both regions have increasingly engaged in an interregional dialogue on different issues of interest, including terrorism. Here again, the criteria/conditions and the level of mutual trust developed between the two regions will be examined.