This study examines the alignment behaviour of ASEAN towards external parties between 1967 and 1999 with the goal of establishing which factors have been most important for determining patterns of cooperation and conflict in Southeast Asia. For this purpose, the study uses an integrative perspective that primarily builds upon the insights from realism and social constructivism but also incorporates some ideas borrowed from selective cognitive theories. The study pursues three goals. Firstly, on the theoretical level, it aims at making a contribution to debates in security studies about the causes of alignment. In particular, this study assesses the relevance of three different explanations of alignment in the ASEAN context: balance-of-threat theory, identity-based accounts, and balance-of-interest theory. Secondly, it evaluates to what extent ASEAN has evolved into a regional community, based on the existence of a collective identity. The notion of regional community is subjected to critical analysis by gauging ASEAN diplomacy against three indicators that are used to operationalize the concept of regional identity: shared problem representations, mutual identifications and norm compliance with the 'ASEAN way'. Thirdly, with regard to the study of international relations in Southeast Asia, this study intends to explore the nature of state interaction in the region. What factors have guided alignment of ASEAN members between 1967 and 1999? The findings of this study are threefold. Firstly, only limited support is found for both threat-based and identity-based explanations of ASEAN alignment, whereas balance-of-interest theory has high plausibility for most cases of ASEAN alignment. Secondly, with regard to the community idea, this study concludes that while ASEAN's diplomacy has partly been based on the idea of community, ASEAN has constituted a rule-based community, not one based on a shared regional identity. Thirdly, as to the nature of state interaction in the region, this study finds that, on a fundamental level, ASEAN behaviour has been characterized by continuity since 1967: alignment has been in support of the status quo. Moreover, security relations in Southeast Asia have become characterized by a high degree of ambiguity, yet this ambiguity has not been adequately accounted for by Western security theory.