After more than a decade of the euro, we have a good idea about what sort of European macro-economic governance structure is in the making — its underlying principles and its relationship to EU 'deepening' and 'widening' and, not least, to globalization. In relation to economic union, above all the single market, the principle of unitary integration provides an overarching framework, its rationale provided by customs union theory. In contrast, monetary union exhibits the principle of differentiated integration, its rationale founded on 'will and capability'. This duality is expressed in the EU as 'one market' co-existing with several currencies, one of which is the euro. We conceptualize the outcome as 'transverse integration' to capture the hybrid, multidimensional and dynamic character of European macro-economic governance. This concept seeks to show that European macro-economic governance transcends traditional classifications such as euro 'insiders' versus 'outsiders' and 'frontrunners' versus 'laggards' in euro entry. It offers a tool with which to critically examine these classifications. Transverse integration is also multidimensional in capturing the input side (public opinion), governance structures ('institutional fuzziness'), and output side (performance) of European macro-economic governance. Finally, it highlights the dynamic character of European macro-economic governance, showing how the balance between the unitary and differentiated integration principles changes over time. The article argues that it makes more sense to study unitary and differentiated integration as cross-cutting phenomena in European macro-economic governance. Experience suggests that differentiation should not be seen as a temporary phenomenon and hence that transverse integration is here to stay.