Abstract In this article the author assesses the proportionality principle in EU law from a legal theoretical and constitutional perspective with the aim of discovering the function of the principle. Having first discussed the implications of the proportionality principle being a general principle of law, and what function it has—namely to secure legitimacy for judicial decisions—the author suggests that there are several ways in which the principle can be interpreted. There is, nevertheless, a limit to this interpretation determined by the proposed function of the principle. In the third part of the article, the European Court of Justice's (ECJ's) interpretation of the principle is assessed. The assessment clearly shows that the ECJ is interpreting the principle in different distinguishable ways. The question could, however, be raised as to whether the ECJ in some areas is interpreting the principle in a way that undermines the very function of it.