This article examines the Member States’ new obligations under DAC-6, which requires the enactment of mandatory disclosure rules for potentially aggressive cross-border tax-planning arrangements and analyses their compatibility with EU primary law. After describing the tax policy environment in which the DAC-6 rules were introduced, Part 1 of this article – while providing a brief overview of the mandatory disclosure rules – discusses whether the Treaty on the Functioning of the European Union foresees a viable legal basis for harmonization with respect to this subject matter and examines whether the DAC-6 rules are in line with the fundamental freedoms. Part 2, to be published in issue 7 of European Taxation (2019), continues with an in-depth analysis of DAC-6’s potential interference with the rights enshrined in the Charter of Fundamental Rights of the European Union and closes by highlighting selected tax policy issues associated with the mandatory disclosure regime of DAC-6.