Customary International Law Norms

Doctrine International Law Preferred

Binding rules arising from general State practice accepted as law, independent of treaty texts.

Independent referenceNot an official or legal definition
Sources citedSee the sources at the end of this page
Status Under reviewLast updated 11 October 2026
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Definition

In plain language

Customary norms are unwritten rules that bind States because States have long and widely acted in a certain way and believe they are legally required to do so.

Academic definition

Customary norms are identified by two constituent elements, a general practice and acceptance of that practice as law (opinio juris), as stated in Article 38(1)(b) of the ICJ Statute and in the ILC's 2018 Conclusions, and applied in North Sea Continental Shelf (1969) and Nicaragua (1986). The ICJ in Nicaragua found that customary rules continue to exist alongside treaty rules of identical content, so a treaty does not displace custom. Particular customs among limited groups of States, as in the Asylum case (1950), require proof of acceptance by the parties concerned, and a persistent objector may avoid being bound by an emerging general rule.

Classification

Term type
Doctrine
Category
International Law
Subject field
International Law
Subfield
Sources of International Law
Difficulty level
Advanced
Grammatical category
noun phrase

Language variants

Alternative terms
  • customary law norms
  • rules of customary international law
Non-preferred terms
  • not_applicable
Abbreviations
  • not_applicable
Acronyms
  • not_applicable
Native terms
  • normes coutumières (French)
Transliteration
not_applicable
Pronunciation
not_applicable

Context

Legal effect: Binding

Scope

Covers the category of customary norms and how they are identified; individual disputed rules, general principles and treaty obligations are separate, and the wording differs from the 'Certain Rules of Customary Law' record.

Legal context

ICJ Statute Article 38(1)(b); ILC Draft Conclusions on Identification of Customary International Law (2018), noted by the General Assembly; ICJ decisions in North Sea Continental Shelf (1969), Nicaragua (1986) and Jurisdictional Immunities of the State (2012). Peremptory norms are a special category of custom that cannot be derogated from.

Diplomatic context

States cite customary law to bind non-parties to treaties and to challenge claims, and their official statements form part of the evidence of practice.

Institutional context

Identified by the ICJ, other courts, the ILC, the ICRC and State legal advisers, and discussed in the UN Sixth Committee.

Historical context

Custom was the main source of international law before large-scale treaty-making in the nineteenth and twentieth centuries, and remains central for general rules.

Why it matters

Customary norms bind States regardless of treaty membership, covering areas such as diplomatic immunity, the use of force and parts of the law of the sea.

Usage

Register: Legal

Usage note

Use 'customary international law' as the general term and 'customary norms' when referring to individual rules.

Example

The court found that the prohibition had crystallised as a customary norm binding on all States.

Example scenario

A State has not ratified a convention on the treatment of prisoners. A tribunal examines official statements and military manuals from many States, finds consistent practice and legal conviction, and holds the rule binding on that State as custom.

Common misuse

Treating a frequent practice as custom without evidence that States regard it as legally required.

Not to be confused with

  • General principles of law
  • Treaty obligation
  • Soft law
  • Usage (without legal conviction)

Sources

  1. Statute of the International Court of Justice, International Court of Justice, Article 38(1)(b). icj-cij.org
  2. Official definition. not_found_in_source
  3. Treaty reference. icj-cij.org
  4. International Law Commission, Draft Conclusions on Identification of Customary International Law (2018). legal.un.org
  5. not_found_in_source. not_found_in_source

Source type: Treaty. Source reliability: Primary.

This entry is an independent reference definition, not an official or legally binding text. Where an official definition exists, it is linked above and is authoritative. Read our methodology.

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