The Court of Justice of the European Union is a judicial body whose decisions play an important role and have an impact on the legal systems of the Member States and their functioning. It is the decisions of the Court that ensure European Union law is effective and consistent in all the Member States of the European Union. Due to emerging global challenges that constitute a denial of traditional democratic values, and as a result create space for the development of populism and illiberal forms of democracy, the chapter will focus on indicating what role the Court of Justice of the EU currently plays in protecting the fundamental principle of the rule of law. In order to be able to answer this pressing question, the case of the constitutional crisis that began in Poland in 2015 and continues to this day will be analysed. This example will be used to discuss how a Member State violated the treaty (and constitutional) rule of law. Then, the individual actions taken by the Court of Justice of the European Union and the judgments it issued to restore the rule of law in Poland will be described and finally, their effectiveness will be assessed.