This chapter provides a comprehensive examination of the concept of the Rechtsstaat, or State of Law, tracing its historical roots, theoretical evolution, core principles, and practical relevance especially in post-Soviet contexts. The chapter begins with a detailed historical account, highlighting the contributions of thinkers from Solon and Aristotle to Locke, Kant, Montesquieu, and beyond, who shaped the idea of law-bound statehood, grounded in justice, individual rights, and the limitation of power. It distinguishes Rechtsstaat from mere legalism, emphasizing that legality alone is not sufficient: laws must be just, fair, and serve human dignity. The chapter then presents key elements that define a Rechtsstaat: rule of law, supremacy of human rights and freedoms, mutual responsibility between state and citizen, separation of powers, high legal culture, and the essential role of civil society. It underscores that civil society is not only a component but a precondition for the realization of a true State of Law, providing both accountability and participation in public life. In conclusion, the chapter reflects on the challenges and prospects for post-Soviet states aspiring to build functioning Rechtsstaaten, arguing that this ideal demands more than constitutional declarations—it requires deep legal culture, just legal frameworks, vibrant civic engagement, and long-term commitment to democratic principles.