This paper analyses the current and potential impact of Southeast Asia's regionalism on the European Union. It begins by giving an overview of the different manifestations that this regionalism takes (ASEAN and AFTA, sub-regional economic zones, APEC) and comments on the overlapping linkages between them. The EU's stake in Southeast Asia is then discussed in the context of broadening the EU's interregional relations with East Asia. A detailed evaluation of the opportunities and threats that regionalist developments in Southeast Asia pose to the EU is presented thereafter. It is argued that the balance of effects will vary less for “insider” EU firms, which have established operations within ASEAN, and more for “outsider” EU firms. The potential benefits the former anticipate from Southeast Asian regionalism are considerable.