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International Law Research Guide: Int'l Law Defined

A research guide to help researchers locate and understand public and private international law resources.

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General Principles of Law

Jus Cogens

Soft Law

What is International Law?

Public International Law is the law of the political system of nation-states. It is a distinct and self-contained system of law, independent of the national systems with which it interacts, and dealing with relations which they do not effectively govern.

Since there is no overall legislature or law-creating body in the international political system, the rules, principles, and processes of international law must be identified through a variety of sources and mechanisms. This can make international law appear difficult to pin down.

Students and scholars in the United States often use the Restatement of the Law (Third), the Foreign Relations of the United States as a guide to identifying international law as applied in the US.

Private International Law is the body of conventions, model laws, national laws, legal guides, and other documents and instruments that regulate private relationships across national borders.

Identification of Authoritative Texts

The Charter of the United Nations establishes the International Court of Justice (ICJ) as the principal judicial organ of the UN. The treaty which establishes the ICJ is known as the Statute of the International Court of Justice.  Article 38 of the "Statute" furnishes an indirect answer to the question: What are the texts of international law? The article is written in terms of what sources the court will use in order to resolve a dispute. These sources include treaties, customary law, case law, academic writings, and general principles of law. Article 38 reads:

1. The Court, whose function is to decide in accordance with international law such disputes as are submitted to it, shall apply:

a. international conventions, whether general or particular, establishing rules expressly recognized by the contesting states;

b. international custom, as evidence of a general practice accepted as law;

c. the general principles of law recognized by civilized nations;

d. subject to the provisions of Article 59, judicial decisions and the teachings of the most highly qualified publicists of the various nations, as subsidiary means for the determination of rules of law.

2. This provision shall not prejudice the power of the Court to decide a case ex aequo et bono, if the parties agree thereto.


A different presentation of these ideas can be found in the Restatement of the Law 3d: Foreign Relations Law of the United States, Articles 102 (Sources of International Law) and 103 (Evidence of International Law).

§ 102 Sources of International Law

(1) A rule of international law is one that has been accepted as such by the international community of states

(a) in the form of customary law;

(b) by international agreement; or

(c) by derivation from general principles common to the major legal systems of the world.

(2) Customary international law results from a general and consistent practice of states followed by them from a sense of legal obligation.

(3) International agreements create law for the states parties thereto and may lead to the creation of customary international law when such agreements are intended for adherence by states generally and are in fact widely accepted.

(4) General principles common to the major legal systems, even if not incorporated or reflected in customary law or international agreement, may be invoked as supplementary rules of international law where appropriate.

§ 103 Evidence of International Law

(1) Whether a rule has become international law is determined by evidence appropriate to the particular source from which that rule is alleged to derive (§ 102).

(2) In determining whether a rule has become international law, substantial weight is accorded to

(a) judgments and opinions of international judicial and arbitral tribunals;

(b) judgments and opinions of national judicial tribunals;

(c) the writings of scholars;

(d) pronouncements by states that undertake to state a rule of international law, when such pronouncements are not seriously challenged by other states.


Since the adoption of the ICJ statute in 1946, the post World War II growth of a wide variety of Inter-Governmental Organizations (IGOs) has injected the work product of these IGOs into the mix as well.

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