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Since the beginning of the 21st century, European studies have developed an interest in the analysis of “variable geometry Europe”. This introduction proposes a critical dialogue with the mainstream theories of European integration which conceptualise “horizontal” differentiation (territorial variations) and “vertical” differentiation (variations in the centralisation of power from one public policy to another). A third logic of “transversal” differentiation is proposed to capture the professional groups working to build a variable geometry Europe thought as a social fact.