In light of recent developments in the US and elsewhere, the world is asking: What constitutes the Rule of Law, and which principles must not be violated? This chapter and the next provide profound answers, even though the understanding of the rule of law can vary from one jurisdiction to another. When assessing the rule of law as a fundamental pillar of the constitutional order, both its structural and value-based functions require analysis. This chapter emphasizes the essential elements of the rule of law, focusing on the structure and function of law—such as clarity, certainty, and stability; the hierarchy of the legal order; the reservation of formal law; and its interaction with international and supranational law. It underscores the constitution as a functional unit, highlighting the interdependence of fundamental values and distinguishing between its necessary and non-necessary components as the constitutional core. Furthermore, it elaborates on the concept of an authentic constitution, with the rule of law as an overarching principle, and examines its internal application, extraterritorial scope, and implications for societal relationships. In its second part, the chapter places the rule of law in a comparative context, addressing its terminology, methodology, and normative validity across different regions, including Latin and North America, Europe—particularly France and the UK—the Middle East, as well as India, Japan, Korea, and selected African constitutional frameworks. It also explores the rule of law in the non-state era, its significance within the European Union, and the mutual normative interdependence between state and Union levels, along with its impact on the international order. The chapter ought to be read in context with the subsequent chapter: Rule of Law: Specific Elements and Separation of Powers.