This article analyses the ECJ’s use of the margin of discretion technique to adjudicate on fundamental rights. It argues that the margin of discretion operates in a way that incorporates the interpretations of national decision-makers into the ECJ’s formulation of EU fundamental rights. In so doing, the margin of discretion does not uphold boundaries between national and European rights. Rather, it allows the national decision-maker and the ECJ to co-create the standard of protection of fundamental rights. The argument is based on an analysis of case law from the late 1970s to the early 2020s, and accounts for overlooked yet important distinctions between different uses of the technique. It shows that the ECJ’s use of the margin of discretion is not synonymous with the ECtHR’s margin of appreciation doctrine. The article contributes to the debate about the boundary question in EU fundamental rights law, namely the question of how national boundaries are, or should be, manifested.