This chapter unveils the fundamental role of constitutional principles—the core of any constitution. Constitutional principles form the foundation and guiding framework of the constitution, shaping the institutional order, decision-making processes, and the substantive orientation of state actions. Beyond providing guidance for governmental activities and actors, constitutional principles ensure the unity and coherence of the constitution. They also play a crucial role in constitutional interpretation and in addressing normative gaps. For these reasons, an in-depth study of these principles is of immense importance. To properly classify these principles, the chapter examines their normative scope and mode of functioning. It first distinguishes constitutional principles from rules before categorizing them into substantive and formal principles. Substantive principles, such as the welfare state principle, obligate the state to pursue specific objectives, whereas formal principles, such as the separation of powers, define the state’s organizational structures. Particular attention is given to how conflicting principles are handled, especially through the principle of proportionality. Additionally, the chapter discusses judicial procedures for addressing misinterpretations or violations of constitutional principles, as well as the broader role of the judiciary in ensuring their enforcement. The far-reaching normative effects of constitutional principles make it clear that they play a central role in shaping both the interpretation and evolution of the constitution. These principles are not just theoretical concepts—they actively influence governance, legal decisions, and the stability of constitutional systems. Understanding them is essential for anyone involved in law, politics, or public administration.