The international relations literature on regionalism, both in economic and security issues, has grown dramatically over the last 15 years. One of the ongoing issues discussed in most articles and books is the conceptualisation of ‘region’. Instead of thinking about regions using notions of interdependence and interaction we take a social constructivist approach, whereby states themselves define regions via the construction of regional economic institutions (REI). We explore how a conceptualisation of region based on REIs contrasts with various related concepts such as regional system, and regional IGO. Empirically, we show that most all countries belong to at least one important regional economic institution, REI, (for example, EU, Mercosur, ASEAN, etc). In short, the world is dividing itself into regions by the creation of regional economic institutions. We contrast our economic-institutional approach to regions with Buzan and Wæver's ‘regional security complexes’ which is based on security dependence. There are interesting agreements and disagreements between their approach and our economic-institutional approach to defining regions. It is perhaps not surprising that many REIs have taken on security roles, which we briefly show by looking at military alliances embedded in REIs. This suggests that policymakers are creating regions through institutional innovations that link economic and security issues.