This chapter argues that the term ‘European public order’ is vague and indeterminate. As a consequence, the ECtHR’s claims that the ECHR is a ‘constitutional instrument of European Public Order’ can have many meanings, and its application in the case law is problematic. This chapter explains why defining European public order is virtually impossible. Unsurprisingly, the ECtHR has failed to define it or to provide an approximate list of its components, and a clear definition of public order is yet to be developed in legal scholarship. This chapter builds upon the argument made in Chapter 2 that the formula ‘European public order’ is so abstract and open-ended that it can be used to support mutually exclusive arguments. For instance, qualified rights can be limited to protect public order; at the same time, the Court can broaden the scope of those same rights because they are an essential part of European public order. These two options, while in contradiction, could be at stake in the same case. This dilemma illustrates the inherent challenge of vague legal notions to the rule of law and legal certainty.