This article analyses the extent to which European Union (EU) directives allow for variation in domestic implementation. Such flexibility in implementation may be used to deal with heterogeneity among member states. Based on an original dataset of 164 directives adopted between 2006 and 2015, we find that the use of flexibility is associated more with efforts to accommodate differences between national policies (substantive use of discretion) than with attempts to facilitate the decision-making process in and between EU legislative institutions (strategic use of discretion). Although flexibility may be used to address some of the same concerns that drive differentiated integration (DI), the situations in which each is most likely to be used are distinct because they approach the divergences between member states differently.