The thesis provides a theoretically informed understanding of the ongms and development of the diplomatic and security culture of the Association of Southeast Asian Nations (ASEAN). It explores the meaningfulness and relevance of the . ASEAN way' as a framework for political and security co-operation, as seen by regional leaders in Southeast Asia, and examines to what extent interpretations of its core principles and associated practices have evolved over time. These include the principles of sovereign equality, non-intervention and non-interference, quiet diplomacy, mutual respect, and the principle of not involving ASEAN to mediate bilateral disputes among its membership. The origins of the' ASEAN way' are explored in the context of the rise of Asian nationalism as well as experiences of alienation and subsequent reconciliation or accommodation between the governments of the founding members. Its development is analysed against the backdrop of changes in the strategic environment in East Asia and challenges to its core principles and practices. As regards indirect and direct intramural challenges the thesis explores the nature of proposals to attain a political solution of the Cambodia conflict in the 1980s, especially that of 'cocktail diplomacy', and Thailand's 1998 proposal for 'flexible engagement', put forward in the wake of the Asian financial crisis. In relation to extramural challenges, the thesis explores to what extent ASEAN has been successful in winning acceptance of its diplomatic and security culture as a framework for regional co-operation in the post-Cold War period by the People's Republic of China and the United States. It also assesses to what extent ASEAN has insisted on or modified the extramural and intramural dimensions of the' ASEAN way' to accommodate the challenges posed by these two regional great powers. TIle thesis argues that while the' ASEAN way' is indeed changing, this change, at least for the moment, focuses mainly on extending the range of issues and contexts traditionally defined as internal affairs in which other ASEAN governments may now legitimately become involved. Considerations about ASEAN cohesion, regime security and regional influence do not suggest that principles and related practices, such as quiet diplomacy or restraint, are no longer relevant to ASEAN decision-makers.