In an era marked by geopolitical fragmentation and open challenges to international norms, the relevance of international law is often questioned. Yet it is precisely under such conditions that international law is most significant. To that end, understanding the institutions that interpret and articulate international law—particularly in moments of strain—is essential. One such institution is the United Nations International Law Commission (ILC), which occupies a unique position in the international legal order.
This post is the second in a multi-part series connected to my work as an adjunct professor at American University, Washington College of Law, where I am teaching an upper-level practicum on the ILC. The aim of this series is to make the institutional role, working methods, and legal significance of the ILC more accessible to readers beyond the classroom.
As always, if there’s something you’d like to see added, clarified, or explored further, I’m happy to hear from you!
Introduction
After establishing the historical context of the International Law Commission (ILC) in the series’ first post, the next step is to establish the Commission’s mandate (or, for non-lawyers, its purpose as intended by those that established the organization).
Article 1(1) of the ILC Statute sets out the Commission’s mandate: “the promotion of the progressive development of international law and its codification.” At first glance, this formulation appears straightforward. In practice, however, the distinction between progressive development and codification has shaped both the Commission’s methodology and the authority of its outputs for more than seventy years. Accordingly, understanding how and why the ILC operates along this dual axis is essential to understanding the Commission’s work and how it affects the evolution of international law.
Progressive Development and Codification: Origins of the Distinction
While drafting the United Nations Charter in 1945, States considered explicitly authorizing the UN to engage in the “revision” of existing international rules. However, ultimately, that language was rejected. Instead, Charter Article 13(1) empowers the General Assembly to “initiate studies and make recommendations for the purposes of . . . encouraging the progressive development of international law and its codification.”
The choice of terminology was deliberate. As the drafting records (also known as “travaux préparatoires” to international lawyers) indicate, the drafters intentionally juxtaposed “progressive development” with “codification” to imply that the Commission could both modify and add to existing rules. In doing so, the drafters created a balance between both continuity and change, rather than laying too significant an emphasis on change alone.
That conceptual framework was carried forward when the General Assembly established the ILC in 1947. The Committee of Seventeen, which created a draft Statute for the Commission, recognized that international lawmaking does not always occur in ideal doctrinal conditions. Instead, the topics on the Commission’s agenda could range from those characterized by limited State practice and sparse legal precedent—i.e., topics requiring progressive development—to those that are heavily developed and well documented—i.e., topics requiring codification. The Commission’s mandate (as set out in Article 1(1) of the ILC Statute) was designed to accommodate both.
Defining the Two Functions
As any good lawyer knows (and all law students learn), the first place of reference when discussing an issue is the relevant legal statute(s) or legislation(s). In this case, the ILC Statute provides formal definitions of both “codification” and “progressive development” that continue to guide the Commission’s work.
Under Article 15 of the Statute:
- Codification refers to “the more precise formulation and systematization of rules of international law in fields where there already has been extensive State practice, precedent and doctrine.”
- Progressive development refers to “the preparation of draft conventions on subjects which have not yet been regulated by international law or in regard to which the law has not yet been sufficiently developed in the practice of States.”
These definitions reflect the conceptual division between lex lata (the law as it is) and lex ferenda (the law as it should be). In this sense, codification is descriptive, addressing lex lata, while progressive development is forward-looking, addressing lex ferenda—a division that plays an important role in the Commission’s work and internal deliberations.
Practical Effect
After providing these definitions, the Statute specifies the different working methods that apply depending upon whether a project is progressive development or codification. Accordingly, Statute Articles 16 and 17 were intended to govern if a project was considered progressive development, and Articles 18 to 24 were intended to govern if a project was considered codification. However, in practice, most ILC projects involve elements of both functions, with the balance varying topic-by-topic.[1] Moreover, the Committee of Seventeen itself recognized that the distinction they themselves created likely would not be mutually exclusive.
Indeed, the formulation of existing law often exposes gaps or inconsistencies that require clarification through proposed rules not yet established in law, while proposals for new rules frequently build upon well-established patterns of State practice and opinio juris.[2] For this reason, the Commission has at times needed to suggest new rules for adoption by States while codifying existing law. An example of this can be seen in the Immunity of State Officials from Foreign Criminal Jurisdiction project, in which Commission members noted that aspects of the procedural framework constituted progressive development that was necessary to facilitate the implementation of provisions codifying existing law.[3]
Why the Distinction Matters
The distinction between progressive development and codification matters for several reasons.
First, it affects how States receive and respond to the Commission’s outputs. Draft articles presented as codification may be treated as authoritative restatements of existing law, while those framed as progressive development may attract greater scrutiny or resistance.
Second, it shapes the legal weight of the Commission’s work in judicial and arbitral settings. Courts frequently rely on ILC commentaries to identify international law, but they do so with an awareness of whether the Commission viewed a provision as reflective of existing practice or as a proposal for the evolution of international law.
Finally, the distinction raises deeper questions about legitimacy and authority. Progressive development is not binding in itself. If proposals for new rules fail to gain traction in State practice, they may be rejected or even lead to regressive State action that ultimately prevents the proposed rule from solidifying into a binding rule of international law.
Importantly, Commission members keep these implications in mind when characterizing proposed provisions as “progressive development” rather than “codification”.
Conclusion
The Commission’s dual mandate positions it as both a stabilizing and adaptive institution. By codifying existing law, the ILC contributes to legal certainty and coherence. Then, by engaging in progressive development, it creates space for reasoned legal innovation without displacing State consent. Importantly, progressive development acts to prevent the stagnation of international law—enabling it to respond to new forms of conflict, technological change, or shifts in global governance.
Ultimately, the Commission’s enduring significance lies in how it manages the tension between stability and change, skillfully navigating between codifying what international law is and proposing what it ought to become. By navigating this tension deliberately and transparently, the ILC helps preserve international law as a functioning legal system rather than a collection of isolated norms. Moreover, in fulfilling this dual mandate, the Commission sustains international law as an ordered and evolving legal system, rather than a static catalogue of rules.
[1] “A single consolidated procedure based on [the ILC’s] Statute has been evolved by the Commission out of the need to incorporate elements of both lex lata and lex ferenda in the rules to be formulated.” Yearbook of the ILC, 1968, vol. II, p.240, para. 41.
[2] For those new to international law, these are the two elements necessary to establish a principle of “customary international law” (a binding source of international law that is not previously written down). The latter, opinio juris, means evidence that States believe the particular action is required by law.
[3] See A/CN.4/SR.3707, page 4.
Course Materials
References
International Law Commission, Website: https://legal.un.org/ilc/ilcintro.shtml, https://legal.un.org/ilc/work.shtml, https://legal.un.org/ilc/methods.shtml
Additional Reading
- Arnold N. Pronto, Codification and Progressive Development of International Law: A Legislative History of Article 13(1)(a) of the Charter of the United Nations, 13 FIU L. Rev. 1101 (2019), https://doi.org/10.25148/lawrev.13.6.11.
- Sean Murphy, Codification, Progressive Development, or Scholarly Analysis? The Art of Packaging the ILC’s Work Product, GWU Legal Studies Research Paper No. 2012-109.
- Mr. Manuel Rama Montaldo, Conceptual and historical aspects of the process of codification and progressive development of international law in the United Nations, with particular reference to the work of the International Law Commission (Video, UN Audiovisual Library of International Law), https://legal.un.org/avl/ls/Montaldo_IL.html.
Ginger Without Borders 
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