This thesis assesses the new ASEAN Comprehensive Investment Agreement (ACIA) in the light of international practice. Investment protection is at the heart of this investment regime. Considering the ACIA as a tool of regional integration, its structure and contents demonstrate its ultimate objective of attracting intra-ASEAN investment flows for the realisation of a single market and production base under the ASEAN Economic Community (AEC). Analysis focuses on the specific elements of the ACIA and how they balance two contradictory interests, i.e. the protection of ASEAN investors and the sovereignty of ASEAN Member States to regulate investments in their territory. Tracking the solutions and innovations of substantial and procedural provisions introduced by the ACIA, it is found that the “ASEAN Way” of consensus and flexibility remains, even though ASEAN has become the AEC with rules and institutions. This general ASEAN Way is specifically reflected in the “ASEAN Way of Investment Protection”. Given the ASEAN-specific context, the ACIA shows a unique balance of States’ and investors’ interests which differs from that of international investment agreements of other regional integration initiatives. While the ACIA aims to protect ASEAN investors, it attempts to respect the sovereignty of ASEAN Member States, by giving more policy space to regulate for public purposes. From these findings, recommendations are offered to improve the ASEAN investment regime. The understanding of the “ASEAN Way of investment protection” may help interpretation and application of investment protection standards of the ACIA, as well as the other existing investment agreements. The ACIA may also serve as a platform for negotiations of future ASEAN investment agreements.