This article analyses how the ECJ applies the proportionality principle in respect of direct taxation, as well as the consequences flowing therefrom. Part 1 addresses the proportionality principle and its application by the ECJ, while Part 2, to be published in European Taxation 10 (2017), covers the impact of the proportionality principle on justifications and the sovereignty of Member States in direct tax matters, as well as the consequences of the ECJ’s approach towards proportionality.