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In international law, codification refers to the formal restatement of existing customary international law into written, structured instruments — usually multilateral treaties. It is distinguished from progressive development, which involves drafting new rules where state practice is unsettled, although the two processes often overlap in practice.
The United Nations Charter, in Article 13(1)(a), tasks the General Assembly with initiating studies and recommendations to encourage "the progressive development of international law and its codification." To carry out this mandate, the GA created the International Law Commission (ILC) in 1947 (Resolution 174(II)), composed of independent experts elected by the Assembly. The ILC drafts articles, circulates them to states for comment, and forwards finalized texts to the GA, which may convene a diplomatic conference to adopt them as a treaty.
Landmark codification treaties drafted with ILC involvement include:
Codification serves several functions: it clarifies ambiguous customary rules, makes the law accessible to non-specialist states, reduces disputes over the content of obligations, and creates a written reference point for courts and tribunals. However, codification can also "freeze" customary law at a particular moment, and unratified codification treaties may still influence practice as evidence of opinio juris.
At the domestic level, codification refers to the consolidation of statutes and case law into comprehensive codes — the French Code civil (1804) and the German BGB (1900) being the canonical examples in civil-law systems.
Codification restates existing customary law in written form, while progressive development drafts new rules in areas where state practice is unsettled or insufficient. The ILC's mandate covers both, and in practice most draft articles blend the two.
Not as treaty law, but provisions that accurately reflect customary international law remain binding on all states on that customary basis. The ICJ has, for example, treated many articles of the 1969 VCLT as declaratory of custom.
The International Law Commission, established by the General Assembly in 1947, is the principal expert body. Its draft articles are debated in the GA's Sixth (Legal) Committee before any diplomatic conference is convened.