Federalism and the associated multi-level polity-structure have been frequently blamed for delaying the implementation of European Union (EU) directives. However, this verdict is incomplete as only a few studies open the “black box” of federalism to analyse the involvement of subnational parliaments and executives or second chambers in policy implementation. This article fills this gap and explains the transposition delay on the level of each individual implementation measure. Our novel data set covers about 850 directives and the corresponding 1,950 implementation measures between 1990 and 2018 in Germany. Using logistic regression models, we find that involving the subnational authorities substantially delays transposition. Subnational measures are three times more likely to be delayed than national ones. The effect of the veto power of a second chamber remains inconclusive. Our findings highlight the challenges federalism poses for the multi-level implementation of EU policies and have implications for the broader literature on compliance with public policies.