In this chapter, I first examine how the rule of law has been defined in legal theory, and how it has been distinguished from the rule by law, which is a distortion thereof (Section 3.1). Second, I assess how the rule of law has been conceptualised in the context of the European Union, as this book focuses primarily on the EU legal order (Section 3.2). In this regard, I also draw on the acquis of the Council of Europe. The Council of Europe is a distinct jurisdictional order, yet it heavily influenced the ‘EU’ conceptualisation of the rule of law, and the EU regularly relies on Council of Europe sources in its own legal practices. Finally, I draw on these findings to identify the rule of law’s core principles and to distil the concrete requirements that public authorities must fulfil to comply therewith (Section 3.3). Identifying these requirements – and the inherent challenges to achieve them – will subsequently allow me to build a normative analytical framework that I can use as a benchmark in Chapter 4 to assess how algorithmic regulation impacts the rule of law.