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Jessup Competition Resources: 2026: Customary Law

What is Customary International Law?

Customary international law is particularly important as a source of international law in absence of a treaty or other controlling rule. J. L. Brierly, in The Law of Nations: an Introduction to the International Law of Peace states that in order to determine what international customary law is on a particular subject "we must look at what states do in their relations with one another and attempt to understand why they do it, and in particular whether they recognize an obligation to adopt a certain course, or, in the words of Article 38, we must examine whether the alleged custom shows ‘a general practice accepted as law."

Ian Brownlie, in Principles of Public International Law lists the following sources of custom as "diplomatic correspondence, policy statements, press releases, the opinions of official legal advisers, official manuals on legal questions, e.g. manuals of military law, executive decisions and practices, orders to naval forces etc., comments by governments on drafts produced by the International Law Commission, state legislation, international and national judicial decisions, recitals in treaties in the same form, the practice of international organs, and resolutions relating to legal questions in the United Nations General Assembly."

Selected Treatises

The Changing Nature of Customary International Law

This book examines the evolution of customary international law (CIL) as a source of international law. Using the International Criminal Tribunal for the former Yugoslavia (ICTY) as a key case study, the book explores the importance of CIL in the development of international criminal law and focuses on the ways in which international criminal tribunals can be said to change the ways in which CIL is formed and identified. In doing so, the book surveys the process and substance of CIL, as well as the problematic distinction between the elements of state practice and opinio juris. By applying an inclusive　positivist approach, Noora Arajärvi analyses the methodologies of identification of CIL in selected cases of the ICTY, and their normative foundations. Through examination of the case-law and the reasoning of courts and tribunals, Arajärvi demonstrates to what extent the court's chosen method of identification of CIL affects the process of custom formation and the resulting system of norms in general. The book will be of great value to researchers and scholars of international law, international relations, and practitioners with interests in customary international law.

The Theory, Practice and Interpretation of Customary International Law

Although customary international law (CIL) has been central to international law from its inception, it is often misunderstood. This edited volume remedies that problem by tracing the history of CIL and provides an in-depth study of its theory, practice, and interpretation. Its chapters tackle the big questions which surround this source of international law such as: what are the rules that regulate the functioning of CIL as a source of international law? Can CIL be interpreted? Where do lines between identification, interpretation, application, and modification of a rule of CIL lie? Using recent developments, this volume revisits old debates and resolves them by proffering new and innovative solutions. With detailed examples from international and national courts, it places CIL in a range of settings to explain, explore and reflect upon this developing and highly significant field. This title is also available as Open Access on Cambridge Core.

Reexamining Customary International Law

Reexamining Customary International Law takes on the complex issues and controversies surrounding the history, theory, and practice of customary international law as it reexamines customary law's increasingly important role in world affairs. It incorporates the expertise of distinguished authors to probe many difficult issues that remain unresolved concerning the doctrine of customary law. At the same time, this book engages in a profound exploration of the practical role of customary international law in a variety of important fields, including humanitarian law, human rights law, and air and space law.

Customary International Law and Its Interpretation by International Courts: Volume 3

It is notorious that international courts and tribunals have greatly contributed to the development of customary international law (CIL) by, for instance, articulating the constituent elements of custom and clarifying the conditions required for its modification. This volume demonstrates that they have also been actively engaged in the interpretation of CIL. In elucidating CIL interpretation before and by international courts and tribunals, the volume chooses three focal points: theory, method and normative interactions. Viewing CIL and its interpretation from these vantage points leads to a more complete picture of the role and function of CIL interpretation in international courts. The volume encourages readers to question orthodox theories on CIL and its interpretation, to look anew at what has long been labelled mere identification of custom, and to take a systemic approach to CIL, which, even in the process of interpretation, remains unwaveringly connected to treaties and general principles of law.

Peremptory Norms in International Law

This monograph analyzes the questions raised by the legal effects of peremptory norms of international law (jus cogens). A comprehensive study of this problem has been lacking so far in international legal doctrine. Peremptory norms, although often criticized and even more often approached with skeptical nihilism, nevertheless attract growing doctrinal and practical attention and have increasing importance in determining the permissible limits on the action of State and non-State actors in different areas. In view of this overriding impact on what might otherwise be instances of the law-making process, peremptory norms concern a constitutional aspect of international law. Peremptory norms are non-derogable norms, and the concept of derogation is among the key concepts analyzed here. Derogation from peremptory norms can be attempted in a wide variety of situations, but if peremptory norms are to operate as norms and not merely as aspirations they must generate consequences that are also peremptory. This effects-oriented character of peremptory norms is examined in a variety of fields. Despite the growing relevance of peremptory norms in practice, doctrine has failed to treat the issue comprehensively and has often been limited to examining specific aspects of the problem, such as the impact of peremptory norms in the law of treaties. This fresh effort to examine and explain the phenomenon of peremptory norms in key areas fills an important doctrinal gap through presenting in a systematic way the effects of peremptory norms and reappraising the significance of such effects, bearing in mind their overall nature. It also demonstrates that the hierarchical superiority of peremptory norms is not limited to the sphere of primary legal relations but becomes most crucially relevant after a specific peremptory norm is breached. A norm's peremptory character is relevant not only for its substance but also for its consequences; peremptoriness consists primarily in the capacity to impact through its effects upon conflicting acts, situations and agreements.

Sources of Evidence

Foreign Relations

Evidence of State practice is found in a variety of primary source materials. One of the best sources of State practice is the records of a State’s foreign relations and diplomatic practices. However, while it is tradition to produce such publications, the information can be quite difficult to obtain because many states do not make it available to the public. Below is a selection of websites that may be useful when searching for Foreign Ministries and Delegations to International Organizations.

For the U.S., the best source is the series called Foreign Relations of the United States (1861–), which “presents the official documentary historical record of major U.S. foreign policy decisions and significant diplomatic activity.” The volumes contain documents from Presidential libraries, the Departments of State and Defense, the National Security Council, the Central Intelligence Agency, the Agency for International Development, and other foreign affairs agencies. It also contains the private papers of individuals involved in formulating U.S. foreign policy.

International Organizations 

As described in the Restatement, “The practice of states that builds customary law takes many forms and includes what states do in or through international organizations.”  (Restatement of the Law, Third, the Foreign Relations Law of the United States §102, Reporters’ Notes, at 2.) In particular, the Restatement refers to resolutions and other documents of the United Nations as evidence. Some examples could be voting records, resolutions, declarations, legislative acts, and other documents produced by the organization. The following are sources of official documents: 

Below are additional resources for conducting research into United Nations:

 

National Law and Judicial Decisions

A country’s domestic law (legislation, court decisions, regulations, etc.) are sources of state practice to the extent that they might seek to describe the country’s practice and obligations under international law. National courts can be called upon to decide questions of whether a particular state action has risen to the level of customary law. ONU has a research guide on International & Foreign Legal Research that offers more details on locating foreign law. 

Before moving on, it is worth highlighting the many excellent research guides available in the foreign law section of GlobaLex. GlobaLex is a free online legal publication dedicated to international and foreign law research and published by the Hauser Global Law School Program at NYU School of Law.

Yearbooks

Many countries publish an international yearbook that contains summaries or overviews of national legislation and case law dealing with that State’s international obligations. These yearbooks may include cases involving questions of public international law, legislation concerning matters of international law, practice in international law, and treaty actions. They also include citations to official documents and sometimes include indexes and full-text documents. They often have bibliographies of books and articles published in that country on international law. At this point, you will not be surprised to learn that most yearbooks lag years behind.

Intergovernmental organizations publish yearbooks that provide an annual survey of the organization’s activities as well as State practice. A commonly used yearbook is that of the United Nations International Law Commission, Yearbook of the International Law Commission.

HeinOnline’s Foreign & International Law Resources Database is also a great resource, which includes in-depth publications of the American Society of International Law and prominent Yearbooks from around the world.

Digest of U.S. Practice in International Law 

Digests aid in locating customary international law contained in diplomatic papers of States.

One of the most accessible digests is the digests for the United States. The Office of the Legal Adviser publishes the annual Digest of United States Practice in International Law. to provide the public with a historical record of the views and practices of the Government of the United States in public and private international law covering 1989 to the present. The introduction notes, “The State Department publishes the official version of the Digest exclusively online to make U.S. views on international law more quickly and readily accessible to our counterparts in other governments, and to international organizations, scholars, students, and other users, both within the United States and around the world.”

Secondary Sources

Secondary sources are one of, if not,  the best source for locating evidence of customary international law. Scholarly articles often dig deep into policy considerations and historical content with extensive footnotes to primary sources. Moreover, these extensive footnotes usually provide excellent citations to the documents themselves and sometimes excerpts or texts of the document are needed.

Following is a list of useful secondary sources:

Probably the best resource for U.S. legal researchers is the Restatement of the Law, Fourth, Foreign Relations.  Like all of the Restatements, it is the distillation of the best thinking of scholars, judges, and lawyers. Though it is unofficial, it possesses great authority with a focus on the U.S. and also cites other international documents. If you do nothing else, go to this Restatement to gain a better understanding of customary international law. The Restatement is a reliable source of context, and it is designed to be a rational explanation of the field and can be found in the ONU catalog and on Westlaw.