This chapter provides a comprehensive examination of the diverse sources from which law derives its authority, structure, and guiding principles, especially within the complex legal cultures of post-Soviet states. It begins with a broad, pluralistic definition of “source of law” that includes not only formal state-issued instruments but also societal customs, judicial precedents, scholarly doctrine, and general moral principles. The chapter explores how legal custom, emerging from longstanding social practices, becomes a binding legal norm once recognized by the state. Judicial precedent is addressed as a means of ensuring consistency and adaptability, particularly in common law systems, with an emphasis on how precedent could be cautiously integrated into civil law traditions. Juridical doctrine is presented as an influential interpretive tool, often bridging the gap between codified norms and evolving legal needs. Normative legal acts including constitutions, statutes, and by-laws, are acknowledged as the dominant legal source in Romano-Germanic and post-Soviet systems, but with caution against overreliance and positivist rigidity. The chapter also emphasizes the crucial but often underused role of general legal principles such as justice, equality, and fairness, as both sources of law and interpretive tools, especially important where written law is silent or inadequate. Finally, religious texts are recognized as valid sources of law within faith-based legal traditions, while remaining formally excluded in secular legal systems like those of post-Soviet countries. The chapter argues for a more balanced and inclusive legal culture that acknowledges the complementary roles of all these sources in building responsive and legitimate legal systems.