This thesis investigates Sino-Japanese relations and the post-Cold War security order in Northeast Asia. In particular, it asks whether a ‘security regime’ now exists in the region. Security analysis of Northeast Asia has often focussed upon the likely effects of changes in material power. This has led to predictions of a ‘Back to the future’ scenario of rivalry and possible war. While acknowledging the value of this approach, I question whether it is sufficient; other approaches, notably an investigation of normative changes, are required. In considering both material and non-material factors, I follow the precepts of RSCT – which view RSCs as essentially social constructions. Thus, I employ RSCT’s eclectic posture, exploring three distinctive approaches to the possibility of structural change – Waltz and neorealism, Wendt and social constructivism, and Buzan and the English school. Thus, while not ignoring the impact of shifts in the balance of power on security practices, I also investigate ideational variables – that is the kinds of values, norms and institutions that are shared by the members of the East Asian RSC. I go on to ask why they are shared, how their identities and interests evolve over time and how these changes influence securitisation and desecuritisation practices. By examining these variables through societal, economic and military-political sectors, and locating them at domestic, regional, interregional and global levels, I conclude that, together with Southeast Asia, Northeast Asia has formed a single ‘East Asian security regime’. This conclusion is based upon my interpretation of domestic normative constructions in Japan and in China; the growing regional identity/society in East Asia (especially after the Asian financial crisis); and the increased willingness and ability of regional actors to deal with security challenges. But challenges remain, with recurrent tensions and crises as well as continuing historical mistrust. I believe that, as yet, ideational factors, the shared norms and institutions in the East Asian RSC, are still associated with acceptance of a pluralist Westphalian international society, and these are shared largely instrumentally rather than by genuine belief. Thus, despite enthusiasm for community building, progress has been limited in collective identity formation; and balancing behaviour is still common. This means that, while East Asia has reached at least the lower or middle stages of a ‘security regime’, it is still far away from becoming a ‘security community’.