The European Union (EU) has been through many institutional transformations since the start of the integration project in the 1950s. While much of the literature has focussed on the more dramatic changes, less attention has been paid to instances of more limited institutional change. This article maps out and then accounts for the limitedness of the EU's departure from its original ‘federal blindness’ vis-à-vis regional actors. Theories of institutional change would lead one to expect that, as integration and regionalization heightened, endogenous pressures for change would trigger greater reform than that observed. Using a novel formula to estimate the EU's aggregate regionalization levels over time, the article demonstrates that it peaked between 1986 and 2003 but has since dropped to a level below that of the 1950s. Such a finding not only corrects a widespread assumption about regionalization levels in the European polity, but also provides an explanation for the pace and scope of the observed change as well as predictions about its future sources.
