Sources of International Law
Sources of International Law: Meaning, Types and Article 38 of the ICJ Statute
Key Takeaways
- The sources of international law explain where rules governing relations between States and other international actors originate and how such rules are identified.
- Article 38(1) of the Statute of the International Court of Justice (ICJ) provides the most widely accepted framework for identifying the sources applied in international adjudication.
- The principal sources recognised under Article 38 are international treaties, international custom and general principles of law.
- Judicial decisions and the teachings of highly qualified publicists are described as subsidiary means for determining rules of international law rather than primary law-making sources.
- Customary international law generally requires two elements: a sufficiently general practice and acceptance of that practice as law, commonly called opinio juris.
- Other important materials, including resolutions of international organisations, unilateral declarations of States and jus cogens norms, may also affect the identification, development or operation of international law.
- There is generally no simple hierarchy among treaties, customary international law and general principles, although special rules such as jus cogens occupy a superior position because inconsistent rules cannot validly derogate from them.
What Are the Sources of International Law?
Sources of international law are the recognised processes, materials and legal foundations through which rules of international law are created, identified and applied.
Unlike a domestic legal system, international law does not have a single legislature that makes all laws applicable throughout the international community. International legal rules emerge through several different processes. States may expressly agree to rules through treaties, repeatedly follow practices because they consider themselves legally obliged to do so, or rely upon general legal principles that can operate within the international legal system.
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For this reason, understanding the sources of international law is essential for determining whether a particular international rule exists, who is bound by it and how that rule should be applied.
The most important starting point for understanding these sources is Article 38 of the Statute of the International Court of Justice.
Article 38 of the ICJ Statute
Article 38(1) of the ICJ Statute directs the Court to apply international law when deciding disputes submitted to it.
The provision identifies:
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- International conventions establishing rules expressly recognised by the contesting States;
- International custom, as evidence of a general practice accepted as law;
- General principles of law; and
- Judicial decisions and teachings of highly qualified publicists as subsidiary means for determining rules of law.
Article 38(2) further provides that the Court may decide a case ex aequo et bono if the parties agree.
Although Article 38 technically regulates the law applied by the International Court of Justice, it has become one of the most widely used frameworks for explaining and identifying the sources of international law.
International Treaties and Conventions
What Is a Treaty in International Law?
A treaty is an international agreement governed by international law through which States or other competent international actors undertake legal obligations.
Treaties may be known by several names, including conventions, covenants, protocols, charters, agreements and pacts. The title of the instrument does not by itself determine whether it creates treaty obligations.
Treaties are important because they allow States to create clear and express legal obligations by consent.
Examples include:
- Charter of the United Nations, 1945;
- Geneva Conventions of 1949;
- Vienna Convention on the Law of Treaties, 1969;
- United Nations Convention on the Law of the Sea, 1982; and
- International Covenant on Civil and Political Rights, 1966.
Why Are Treaties Binding?
International treaty law operates on the principle of pacta sunt servanda, meaning that agreements must be performed in good faith.
The Vienna Convention on the Law of Treaties, 1969 provides the principal codified framework concerning the conclusion, interpretation, application, amendment and termination of treaties. The broader law of treaties in international law governs important questions such as the formation of treaties, reservations, validity, interpretation and termination.
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Treaties ordinarily bind only States or other parties that have consented to be bound by them. Consequently, a treaty does not normally create obligations for a third State without its consent.
However, a rule contained in a treaty may also exist independently as customary international law. In such circumstances, even a State that is not party to the treaty may be bound by the corresponding customary rule.
Customary International Law
Customary international law consists of rules derived from a general practice accepted as law.
Article 38(1)(b) of the ICJ Statute expressly recognises international custom as a source of international law.
Determining whether a customary international law rule exists generally requires examination of two elements:
- a general practice; and
- acceptance of that practice as law, known as opinio juris.
Both elements must ordinarily be established.
State Practice
State practice refers to conduct attributable to States.
Relevant forms of State practice may include:
- diplomatic correspondence and communications between governments;
- national legislation relating to international obligations;
- decisions of national courts;
- executive and administrative actions;
- military conduct;
- official statements by government representatives;
- treaty practice;
- voting patterns in international organisations; and
- conduct relating to international disputes.
The practice does not have to be absolutely uniform across every State. However, it must generally be sufficiently widespread, representative and consistent to support the existence of the claimed customary rule.
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The practice of States particularly concerned with a particular subject may also carry importance when determining whether a sufficiently general practice exists.
Opinio Juris
State practice alone does not create customary international law.
States must engage in the relevant practice because they accept that the practice is required or permitted by international law. This legal conviction is described as opinio juris sive necessitatis, generally shortened to opinio juris.
A State may repeatedly perform an act because of political convenience, diplomatic courtesy, morality, tradition or practical necessity. Such conduct does not necessarily establish customary international law unless accompanied by acceptance of its legal character.
The distinction between habitual conduct and legally obligatory conduct is therefore essential in determining whether an international custom exists.
North Sea Continental Shelf Cases
The North Sea Continental Shelf Cases (1969) are among the most important decisions concerning customary international law.
The dispute involved continental shelf delimitation between the Federal Republic of Germany, Denmark and the Netherlands.
The ICJ considered whether the equidistance principle contained in the 1958 Geneva Convention on the Continental Shelf had become a rule of customary international law applicable to Germany, which was not a party to the Convention.
The Court did not accept that the rule had acquired that status in the circumstances of the case. The judgment became particularly significant for explaining the requirements of sufficiently extensive and representative State practice accompanied by recognition of a legal obligation.
It also demonstrated that a treaty rule can potentially develop into customary international law, but the existence of a treaty provision alone does not automatically produce that result.
Nicaragua v United States
Customary international law was also central to Military and Paramilitary Activities in and against Nicaragua (Nicaragua v United States of America), 1986.
The ICJ examined customary rules concerning the prohibition on the use of force, non-intervention and respect for State sovereignty.
The Court concluded, among other matters, that the United States had violated obligations under customary international law relating to non-intervention and the use of force.
The case demonstrates an important feature of international law: treaty obligations and customary international law may exist alongside one another. A rule may therefore continue to apply as custom even where the jurisdictional or treaty position prevents reliance upon a particular treaty provision.
General Principles of Law
What Are General Principles of Law?
General principles of law constitute the third category expressly recognised in Article 38(1) of the ICJ Statute.
They help international courts and tribunals deal with legal questions that cannot be fully resolved through treaties or customary international law and contribute to the coherence of the international legal system.
Historically, many discussions have focused on principles found across domestic legal systems and capable of application at the international level.
Examples commonly discussed in international legal reasoning include principles relating to:
- good faith;
- res judicata;
- responsibility for wrongful conduct;
- procedural fairness;
- reparation; and
- certain principles governing evidence and judicial procedure.
However, identifying general principles requires more than merely finding a familiar domestic rule. It must be determined whether the principle is capable of recognition and application within the international legal order.
Why Are General Principles Necessary?
Treaties and customary law do not necessarily provide an express rule for every legal question that may arise before an international court or tribunal.
General principles can assist in preventing gaps in the legal system and allow international adjudicatory bodies to apply fundamental concepts necessary for the administration of justice.
For example, principles relating to good faith, finality of judgments and procedural fairness may be necessary for the effective functioning of adjudicatory processes even where every procedural detail is not contained in a treaty.
General principles therefore contribute to the completeness and consistency of international law.
Judicial Decisions
Are Court Judgments a Source of International Law?
Article 38(1)(d) describes judicial decisions as subsidiary means for the determination of rules of law.
Judgments do not occupy exactly the same position as treaties or customary international law. Their principal function is to help identify, interpret and explain existing legal rules.
Decisions of the International Court of Justice carry particular authority in international legal reasoning. Decisions of other international courts and tribunals, arbitral bodies and domestic courts may also assist in identifying international law depending upon their relevance and persuasive value.
Judicial decisions are especially important where a court has carefully examined State practice, treaty provisions or general principles and explained how an international legal rule operates.
Is There a Doctrine of Binding Precedent in International Law?
International law does not apply a strict doctrine of precedent to ICJ judgments in the same manner as some domestic common law systems.
Article 59 of the ICJ Statute states that a decision of the Court has no binding force except between the parties and in respect of that particular case.
Therefore, an ICJ judgment does not automatically bind every State or determine every future dispute involving a similar legal question.
Nevertheless, earlier ICJ judgments are frequently relied upon by the Court itself, other tribunals, States and scholars because they contain authoritative explanations of international legal rules.
Judicial decisions therefore have substantial persuasive importance even where they do not formally operate as binding precedent in subsequent cases.
Teachings of Highly Qualified Publicists
Article 38(1)(d) also refers to the teachings of the most highly qualified publicists of various nations as subsidiary means for determining rules of law.
These teachings include respected scholarly works, legal commentaries and academic writings dealing with international law.
Historically, scholarly writings played a particularly important role because State practice and international judicial decisions were less systematically documented and accessible.
Today, greater access to treaties, judgments, diplomatic materials, State documents and international organisation records means that scholarly writings ordinarily perform an explanatory and analytical function rather than independently creating international law.
Their persuasive value may depend upon factors such as:
- the expertise and reputation of the scholar;
- the quality of legal analysis;
- the primary materials relied upon;
- the extent to which opposing views have been considered; and
- the level of acceptance received by the interpretation.
The writings of publicists therefore assist in determining the content of international law but are not generally treated as independent law-making instruments.
Resolutions of International Organisations: Are UN General Assembly Resolutions Legally Binding?
A resolution adopted by an international organisation does not automatically become a binding rule of international law merely because it has received broad support.
For example, most resolutions of the United Nations General Assembly are generally recommendatory rather than legally binding.
However, such resolutions may still have considerable legal significance.
They may:
- provide evidence concerning the legal views of States;
- assist in determining opinio juris;
- reflect an existing customary rule;
- contribute to the development of customary international law; or
- formulate principles that subsequently receive wider legal recognition.
The legal significance of a resolution therefore depends upon factors such as its wording, circumstances of adoption, voting pattern and subsequent State practice.
A resolution supported by a large number of States may provide evidence relevant to the identification of international law, but voting in favour of a resolution does not by itself necessarily establish a customary rule.
Unilateral Acts of States: Can a State Create an International Obligation by a Unilateral Declaration?
In certain circumstances, a State may become legally bound through its own unilateral declaration even without entering into a treaty.
The crucial consideration is whether the State intended its declaration to create a legal commitment.
This principle was prominently recognised by the ICJ in the Nuclear Tests Cases (1974) involving Australia and New Zealand against France.
France had made public statements concerning the cessation of atmospheric nuclear testing. The ICJ recognised that unilateral declarations may create international legal obligations when the State making the declaration intends to be bound.
Good faith plays an important role in this context. Other States must be capable of placing confidence in declarations deliberately made with the intention of creating legal consequences.
Therefore, not every political statement, press statement or diplomatic assurance becomes an international legal obligation. The wording, circumstances and intention behind the declaration must be examined.
Jus Cogens or Peremptory Norms
The doctrine of jus cogens concerns peremptory norms of general international law that occupy a special position within the international legal system.
These are norms accepted and recognised by the international community of States as a whole as norms from which no derogation is permitted.
Unlike ordinary international rules, States cannot contract out of a jus cogens rule by entering into an inconsistent treaty.
Examples widely discussed in connection with peremptory norms include fundamental prohibitions relating to:
- genocide;
- slavery and the slave trade;
- aggression;
- crimes against humanity;
- racial discrimination and apartheid; and
- torture.
The principle of self-determination has also been recognised in international legal materials as having peremptory significance.
Jus cogens is therefore better understood as a special status possessed by certain international legal norms rather than simply another separate source listed under Article 38.
Equity and Ex Aequo et Bono
Equity also has a role in international law, particularly where legal rules require consideration of fairness or equitable principles.
A distinction must be drawn between equity within the law and a decision made ex aequo et bono.
Equitable principles may form part of the application and interpretation of existing international law. Courts may therefore take account of equity where a legal rule itself requires an equitable solution.
Article 38(2) of the ICJ Statute, however, separately permits the Court to decide a dispute ex aequo et bono if the parties agree.
A decision ex aequo et bono allows the dispute to be determined on considerations of justice and fairness rather than solely through strict application of existing legal rules.
Consent of the parties is necessary for the Court to exercise this power.
Primary and Subsidiary Sources of International Law
For study purposes, the sources under Article 38 are frequently divided into primary sources and subsidiary means.
Primary Sources
The principal sources are generally described as:
- international treaties and conventions;
- customary international law; and
- general principles of law.
These sources provide substantive rules capable of regulating international legal relations.
Subsidiary Means
The principal subsidiary means are:
- judicial decisions; and
- teachings of highly qualified publicists.
Their primary role is to assist in determining the existence, meaning and scope of international legal rules.
The distinction is important because judicial decisions and academic writings ordinarily identify or explain law rather than independently creating rules in the same manner as treaties or established custom.
Is There a Hierarchy Among the Sources of International Law?
International law generally does not establish a simple hierarchy under which treaties always prevail over custom or custom always prevails over treaties.
The relationship depends upon the circumstances.
A treaty governs relations between its parties within its scope. A customary rule may govern States independently of treaty obligations. A treaty and customary rule may also contain substantially similar obligations and operate simultaneously.
Where different rules apply to the same matter, principles governing the relationship between norms may become relevant.
One such principle is lex specialis derogat legi generali, which broadly means that a more specific rule may prevail over a more general rule dealing with the same subject.
Another is lex posterior derogat legi priori, under which a later rule may, in appropriate circumstances, affect the operation of an earlier rule.
These principles cannot be applied mechanically and their operation depends upon the nature, parties and subject matter of the relevant rules.
Jus cogens, however, occupies a special position. A treaty conflicting with a peremptory norm cannot validly override that norm.
Therefore, the international legal system contains certain relationships of priority between norms even though Article 38 itself does not establish a general hierarchy among the sources listed in it.
Difference Between Treaties and Customary International Law
| Basis | Treaties | Customary International Law |
| Formation | Created through agreement between parties | Develops through general practice accepted as law |
| Consent | Express consent is normally required | Consent is not expressed through one formal agreement |
| Form | Usually contained in written international instruments | Usually identified from State practice and opinio juris |
| Application | Generally binds parties to the treaty | May bind States more generally when the customary rule exists |
| Evidence | Treaty text provides primary evidence | Requires examination of practice and legal conviction |
| Modification | Governed by treaty law and agreement | Develops or changes through State practice and opinio juris |
Importance of Sources of International Law
The law relating to sources performs several important functions.
First, it determines whether an international legal rule actually exists. Political statements, diplomatic practices and moral expectations do not automatically amount to law.
Second, it helps determine which States or other international actors are legally bound by a particular obligation.
Third, it assists international courts and tribunals in resolving disputes by providing recognised methods for identifying applicable international rules.
Fourth, the law of sources explains how international law can develop despite the absence of a single global legislature with general law-making authority.
Finally, understanding the interaction between treaties, customs, general principles, judicial decisions and other international materials is essential for the study of public international law, including human rights law, international humanitarian law, international criminal law, the law of the sea and State responsibility.
Conclusion
The sources of international law explain the foundations from which international legal rights and obligations arise. Article 38 of the ICJ Statute remains the central framework for understanding these sources and recognises treaties, international custom and general principles of law, together with judicial decisions and scholarly writings as subsidiary means for determining rules.
Among these sources, treaties represent expressly accepted obligations, while customary international law develops from general practice accompanied by opinio juris. General principles help support the coherence and functioning of the international legal system. Judicial decisions and scholarly writings assist in identifying and interpreting applicable law.
International law has also developed beyond a rigid reading of Article 38. Resolutions of international organisations may contribute to identifying customary rules, unilateral declarations may create obligations in appropriate circumstances and jus cogens norms impose overriding limits from which States cannot derogate.
The sources of international law therefore operate together rather than in isolation. Their interaction provides the legal foundation through which international law develops, adapts and regulates relations within the international community.
Note: This article was originally written by Harsh Gupta (B.A. LL.B. Student, University Institute Of Legal Studies, Panjab University, Chandigarh) and published on 21 May 2020. It was subsequently updated by the LawBhoomi team on 17 September 2026.
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