Although economic literature about regional integration is substantial, its definition remains controversial. For instance, it is common nowadays for the terms regional integration and regional economic integration to be used interchangeably in spite of the importance accorded by literature to non-economic factors of integration, particularly to political ones. Inspired on the South American integration project, this paper revisits the theory of regional integration and proposes a novel approach for evaluating regional integration initiatives that include not only political but also physical aspects. More specifically, this article proposes to analyze any regional integration project from three complementary angles: economic integration, political integration, and physical integration. Moreover, it argues that political and physical integration constitute a preliminary, or contemporaneous, step toward economic integration, and not a final stage, as the current debate suggest. In other words, it is argued that a zero-stage in (Balassa, 1961)'s theory of economic integration is needed to enable the long-term sustainability of a regional bloc.