Customary law, also known as the law of nations, applies when no other authority is available to create a law. Article 38 of the ICJ Statute provides that a custom is a general practice accepted as law. Some nations may have statutory or other legal authority to address specific customary law theories within their respective jurisdictions.

Article 1 § 8 of the U.S. Constitution gives Congress power to enact laws related to "Offences against the Law of Nations." Many current statutes provide for U.S. courts to apply customary law under specific circumstances. Two examples of such statutes are the Alien Tort Statute (ATS), 28 U.S.C. § 1350, providing that federal courts have jurisdiction over torts committed in violation of international law, and the Foreign Sovereign Immunities Act (FSIA), 28 U.S.C. § 1605, et seq., which lists several specific circumstances under which federal courts have jurisdiction over a foreign state. Each statute grants U.S. courts jurisdiction over matters arising out of customary international law .