Interregionalism means region-to-region relations. Its relevance lies on two assumptions: that regionalism is a significant mechanism of governance and that regions are outward looking. The fact that both assumptions are contested confers the concept of interregionalism a structural fuzziness. In this chapter we seek to grasp the phenomenon by following a sequential path: we first deal with definitions, types and theory, only then to look into the empirical evidence in search of correspondence between names and facts. By looking into transatlantic interregionalism, we find it as a large umbrella that brings together very diverse groupings of countries under a same, moderately inconsequential, working mechanism: summitry.
