Customary law is law that derives its authority from long-established practice rather than from formal enactment. A norm becomes customary law when it is followed consistently over time and when the community that follows it treats it as binding — not merely habitual but obligatory. The distinction matters: people habitually eat lunch around noon, but no one treats this as a legal obligation. Customary law requires both practice (people do it) and opinio juris (people believe they must).
Customary law was the dominant form of legal ordering for most of human history. Before codification, most communities governed themselves through inherited norms transmitted orally and enforced through community mechanisms — elders, councils, religious authorities, social pressure. The codification projects of the eighteenth and nineteenth centuries — the Napoleonic Code, the common law’s gradual textualization — aimed to replace customary law with state law: written, uniform, centrally administered. They succeeded formally but not completely. Customary norms continue to regulate land tenure, family relations, commercial practice, and conflict resolution in communities worldwide, often alongside or beneath state law.
In international law, custom is one of the primary sources of legal obligation. Customary international law — norms derived from the consistent practice of states accompanied by a sense of legal obligation — governs areas where no treaty exists. The prohibition on torture, the immunity of diplomats, and certain rules of armed conflict are grounded in customary international law. The evidentiary challenge is substantial: proving that a norm is customary requires demonstrating widespread, consistent state practice and subjective belief in obligation, both of which are contested.
The encounter between customary law and state law is one of the central dynamics of legal pluralism. Colonial legal systems systematically subordinated indigenous customary law to colonial state law — sometimes abolishing it outright, sometimes codifying selective versions of it (producing what Mahmood Mamdani calls “customary” law, a colonial construction rather than a genuine indigenous tradition). The politics of recognizing, codifying, or displacing customary law remain live in postcolonial legal systems.
Related terms
- Legal pluralism — the coexistence of customary and state legal orders
- Codification — the project of converting custom into written state law
- Natural law — another non-positivist source of legal authority
- Legitimacy — the traditional authority on which customary law rests
- Legibility — the state’s drive to make customary arrangements administrable