The present collective work "The Principle of the Rule of Law in the Legal Order of the European Union" examines the fundamental principle of the Rule of Law, which, due to its universal and horizontal nature, requires respect from both the Union's institutions in the exercise of their powers, especially when it comes to the protection of individuals, and the Member States, for whose accession it is also a prerequisite. The work consists of 11 contributions that aim not only to capture the Union's acquis regarding respect for the fundamental principle of the Rule of Law, providing the relevant information to the reader, but also to identify the "gray areas" within the framework of European integration, where the Rule of Law is tested, while simultaneously proposing solutions to fill the observed gaps. Finally, this work is accompanied by case law from the Court of the EU, references to a rich foreign and Greek bibliography, and a useful alphabetical index.