Differentiated integration in the European Union (EU) has been primarily discussed and analyzed at the treaty level, whereas lack of systematic data has hampered the examination of secondary-law or legislative differentiation. We present a new data set of differentiation in EU legislation from 1958-2012, a descriptive analysis, and a comparison of the patterns of primary and secondary-law differentiation across time, member states, and policies. We find that differentiation facilitating the accession of new members and constitutional differentiation accommodating the opposition against the integration of core state powers drive both primary and secondary law differentiation. In addition, we find complementarity between differentiation in treaty law and secondary legislation depending on the availability and salience of differentiation opportunities.

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