The sources of international law create the framework for practice in, and interaction with, public international law. Researchers often look to two main authorities in order to identify sources of international law, the Statute of the International Court of Justice External (ICJ Statute) and the Restatement of the Law, Third: Foreign Relations Law of the United States (Restatement Third). These sources of law are further explained and explored in their respective sections of this research guide.

Sources of international law are defined under Article 38 External of the ICJ Statute. Article 38 lists sources of international law for the court to follow when overseeing proceedings between member states of the United Nations, or states that have become parties to the ICJ Statute. A nation-state can accept jurisdiction under the ICJ Statute through several means External. The Statute provides that the court, whose function is to decide in accordance with international law such disputes as are submitted to it, shall apply:

• international conventions, whether general or particular, establishing rules expressly recognized by the contesting states;
• international custom, as evidence of a general practice accepted as law;
• the general principles of law recognized by civilized nations;
• 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.

Another list of the sources of international law can be found in §102 of the Restatement Third, which provides as follows.

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

• in the form of customary law;
• by international agreement; or
• by derivation from general principles common to the major legal systems of the world.

Please note that the Restatement Third was superseded in part in 2018 by a newer edition, the Restatement of the Law, Fourth: Foreign Relations Law of the United States (Restatement Fourth). Sections 101 and 102 of the Restatement Third are not affected by the Restatement Fourth; however, researchers should keep this in mind when navigating these resources. In some instances, researchers may find that the Restatement Fourth appears to be silent on a point, and may need to review the Restatement Third for a fuller understanding of certain topics.