International Custom

International custom is one of the principal sources of Public International Law. It consists of general and consistent State practice that States follow because they regard themselves as legally obliged to do so. Unlike treaties, customary international law does not require a written agreement between States. It develops gradually through the conduct of States accompanied by the belief that such conduct is legally required.

Article 38(1)(b) of the Statute of the International Court of Justice refers to “international custom, as evidence of a general practice accepted as law.” Thus, customary international law is traditionally understood through two essential elements: State practice and opinio juris.

Meaning of International Custom

International custom is a rule of international law that develops from the general practice of States accepted by them as legally binding.

Custom is particularly important because international society does not have a single legislature capable of creating rules for all States. Custom therefore provides a way through which legal rules can develop from the actual conduct and legal convictions of States.

In simple terms:

Customary international law = General State practice + Opinio juris

Both elements are important. Repeated conduct alone does not automatically become a rule of international law.

Legal Basis

Article 38(1)(b) of the ICJ Statute

Article 38(1)(b) identifies international custom as one of the sources that the International Court of Justice applies when deciding disputes in accordance with international law.

It refers to:

“international custom, as evidence of a general practice accepted as law.”

This provision provides the classical formulation of customary international law.

Essential Elements of International Custom

There are two principal elements:

1. State Practice
2. Opinio Juris

State Practice

State practice refers to the actual conduct of States.

Practice may be demonstrated through:

• diplomatic correspondence;
• government statements;
• legislation;
• executive actions;
• military manuals;
• national court decisions;
• diplomatic protests;
• treaty practice;
• official publications;
• conduct during international disputes;
• decisions and voting behaviour in international organisations.

The practice must be sufficiently general and consistent to contribute to the formation of a customary rule.

Opinio Juris

Opinio juris means the belief that a particular practice is followed because it is legally required or permitted under international law.

This element distinguishes a legal custom from ordinary habits or practices.

For example, States may regularly follow a particular diplomatic practice because it is convenient or traditional. That alone does not establish customary international law.

There must be evidence that States regard the practice as legally obligatory or legally authorised.

State Practice and Opinio Juris

Element	Meaning	Main Question
State Practice	Actual conduct of States	What do States do?
Opinio Juris	Legal belief accompanying the practice	Why do States believe they should do it?

Both elements must generally be established before a customary rule can be recognised.

Generality of State Practice

State practice does not necessarily require every State to follow the practice.

The practice must generally be sufficiently widespread, representative and consistent.

Particular attention may be given to States that are especially affected by the subject matter of the rule.

For example, in maritime law, the practice of States with significant maritime interests may be particularly relevant.

Consistency of Practice

Practice should be sufficiently consistent.

If States regularly act in completely contradictory ways, it becomes difficult to establish a customary rule.

However, absolute uniformity is generally not required.

Minor inconsistencies do not necessarily prevent the formation or existence of a customary rule.

The overall practice must be sufficiently coherent.

Duration of Practice

There is no universally fixed minimum period required for the formation of customary international law.

A practice does not necessarily have to continue for centuries.

What matters is the generality and consistency of the practice and the existence of opinio juris.

In appropriate circumstances, customary rules may develop relatively quickly if the relevant State practice and legal conviction are sufficiently strong.

Opinio Juris in Detail

Opinio juris is often the more difficult element to establish.

The same conduct can have different legal meanings.

For example, States may refrain from a particular action because:

• they believe it is legally prohibited;
• they consider it politically undesirable;
• they lack the ability to undertake it;
• they simply have no reason to act differently.

Only the first type of reasoning directly demonstrates opinio juris.

Evidence of Opinio Juris

Evidence may include:

• official statements;
• diplomatic protests;
• legal opinions of governments;
• national legislation;
• pleadings before international courts;
• explanations of votes in international organisations;
• treaty provisions accompanied by statements of legal obligation;
• official military manuals.

Custom and Habit

Customary international law should not be confused with repeated behaviour.

Habit

A State repeatedly acts in a particular manner because it is convenient, traditional or politically desirable.

Custom

A State follows a practice because it considers itself legally required or legally entitled to do so.

Therefore:

Repetition without legal conviction does not automatically create customary international law.

Formation of Customary International Law

Custom develops gradually.

A simplified process is:

State conduct → General and consistent practice → Opinio juris → Recognition of a customary rule

The process is not always linear. States may simultaneously engage in practice and express their legal positions.

Types of Custom

General Custom

General customary international law applies broadly across the international community when the requirements for a general customary rule are satisfied.

Regional Custom

A customary rule may develop among a particular group of States within a region.

Such a rule does not automatically bind States outside that regional practice.

Local or Particular Custom

A customary rule may also develop between a smaller number of States where sufficiently consistent practice and legal acceptance exist.

Persistent Objector

The persistent objector doctrine concerns a State that consistently and clearly objects to the formation of an emerging customary rule from an early stage.

Under the traditional doctrine, a State that persistently objects may, subject to important limitations, avoid being bound by the customary rule once it becomes established.

Requirements

The State generally needs to:

• object while the rule is developing;
• object consistently;
• make its position sufficiently clear.

A State that begins objecting only after a customary rule has already become established cannot ordinarily rely on the doctrine merely because it disagrees with the rule.

Persistent Objector vs Jus Cogens

The persistent objector doctrine does not provide a means of escaping peremptory norms of general international law (jus cogens).

A customary rule possessing jus cogens status cannot be displaced by an individual State’s objection.

Regional and Local Custom

Custom does not always have to be universal.

A particular customary rule may develop among a specific group of States if the necessary elements are established.

The ICJ considered regional custom in the Asylum Case.

The Court emphasised that a State claiming a regional or local customary rule must establish sufficiently consistent and accepted practice.

Custom and Treaties

Treaties and customary international law are closely related.

A treaty may:

1. codify an existing customary rule;
2. crystallise an emerging customary rule;
3. contribute to the development of a new customary rule;
4. establish obligations specifically applicable to its parties.

Therefore, the existence of a treaty provision does not necessarily prevent a parallel customary rule from existing.

Customary Law and Non-Part States

A customary rule can apply to States regardless of whether they are parties to a particular treaty, provided that the rule has independently acquired customary status.

For example, a rule contained in a multilateral treaty may also reflect customary international law.

In such a situation, a State may be bound by the customary rule even if it has not become a party to the treaty.

Custom and General Principles of Law

Custom and general principles of law are separate categories under Article 38 of the ICJ Statute.

Custom

Develops through general State practice accepted as law.

General Principles

Reflect fundamental principles recognised within legal systems and used as a source of international law.

They should therefore not be treated as interchangeable concepts.

Importance of International Custom

Customary international law performs several important functions.

1. Fills Gaps

Custom can regulate matters where no applicable treaty exists.

2. Binds Beyond Treaty Membership

A customary rule can apply more broadly than a treaty provision.

3. Develops International Law

Custom allows international law to evolve in response to changing State practice and international conditions.

4. Supports International Stability

Established customary rules provide predictable standards for State conduct.

5. Reflects International Practice

Custom connects legal rules with actual conduct and legal positions of States.

Important Cases

The Paquete Habana, 1900

The United States Supreme Court recognised customary international law concerning the treatment of coastal fishing vessels.

The case is often discussed as an example of the judicial recognition of customary international law.

S.S. Lotus Case, 1927

The Permanent Court of International Justice considered principles concerning State jurisdiction and the role of international practice.

The case is historically important in discussions of State sovereignty and customary international law.

Asylum Case, 1950

The ICJ considered a claim concerning an alleged regional or local customary rule.

The Court held that the State relying upon the alleged regional custom had to establish that the rule was binding upon the other party through consistent and accepted practice.

The case is important for understanding regional custom.

North Sea Continental Shelf Cases, 1969

The ICJ provided one of the most important explanations of the formation of customary international law.

The Court emphasised the importance of:

• extensive and virtually uniform State practice;
• participation by specially affected States;
• the belief that the practice is rendered obligatory by the existence of a legal rule.

The case is therefore fundamental for understanding State practice and opinio juris.

Nicaragua v. United States, 1986

The ICJ examined customary international law concerning:

• prohibition of force;
• non-intervention;
• self-defence.

The Court confirmed that customary rules can exist alongside treaty provisions.

The case is particularly important for demonstrating that a rule may have customary status independently of a particular treaty regime.

Continental Shelf (Libyan Arab Jamahiriya/Malta), 1985

The ICJ considered the formation and identification of customary rules and emphasised the need to examine State practice and opinio juris.

The case illustrates the continuing importance of actual State conduct in identifying customary international law.

Important Distinctions

Custom vs Treaty

Point	Custom	Treaty
Formation	State practice + opinio juris	Express agreement
Written form	Not necessarily written	Generally written
Consent	Not necessarily express	Express consent to be bound
Evidence	Practice and legal conviction	Treaty text
Scope	Can be general, regional or local	Primarily binds parties
Development	Gradual	Formal conclusion

Practice vs Opinio Juris

Point	State Practice	Opinio Juris
Meaning	Actual conduct	Legal belief
Question	What do States do?	Why do they do it?
Evidence	Conduct, legislation, statements	Legal statements, protests, official positions
Role	Objective element	Subjective/legal element

Custom vs Usage

Point	Custom	Usage
Nature	Legal rule	Repeated practice
Legal belief	Opinio juris required	Not necessarily
Binding force	Legal	May be merely customary/social
Example	Established rule of diplomatic immunity	Repeated diplomatic practice without legal conviction

Common Confusions

Is every repeated State practice customary international law?

No.

Repeated practice must be accompanied by opinio juris.

Does custom have to be written?

No.

Customary international law can exist without a written instrument.

Does every State have to follow a practice?

No.

The practice must generally be sufficiently widespread, representative and consistent.

Is there a fixed number of years required to create custom?

No.

There is no universally prescribed minimum period.

Can a treaty create customary international law automatically?

No.

A treaty provision may contribute to the development or recognition of custom, but customary status must independently satisfy the requirements of State practice and opinio juris.

Can a State escape every customary rule by objecting?

No.

The persistent objector doctrine is limited and does not apply to jus cogens norms.

Is regional custom valid international law?

Yes.

A regional or local customary rule may exist if the required practice and legal acceptance are established among the relevant States.

Relationship with Related Concepts

Custom and State Sovereignty

Custom develops largely through the conduct and legal positions of sovereign States.

Custom and Treaties

Treaties may codify, crystallise or contribute to the development of customary rules.

Custom and Jus Cogens

Some customary rules may acquire the special status of peremptory norms.

Jus cogens norms possess a higher normative status and cannot be displaced by ordinary international agreements.

Custom and International Courts

International courts identify and apply customary rules by examining State practice and opinio juris.

Topic at a Glance

Point	Position
Source	International Custom
Legal basis	Article 38(1)(b), ICJ Statute
Definition	General practice accepted as law
Essential elements	State practice + opinio juris
State practice	Actual conduct of States
Opinio juris	Belief that conduct is legally required or permitted
Duration	No fixed minimum period
Practice	Must generally be widespread and consistent
Types	General, regional and local
Special doctrine	Persistent objector
Leading case	North Sea Continental Shelf Cases
Regional custom case	Asylum Case
Custom and treaties	May coexist and influence each other
Custom and jus cogens	Jus cogens cannot be avoided through persistent objection

Quick Revision

• International custom is one of the principal sources of international law.
• Article 38(1)(b) of the ICJ Statute defines it as international custom, as evidence of a general practice accepted as law.
• Custom has two essential elements:
  1. State practice
  2. Opinio juris
• State practice is the objective element.
• Opinio juris is the legal or subjective element.
• Repeated conduct alone does not create customary international law.
• Practice must generally be sufficiently widespread, representative and consistent.
• There is no fixed minimum period required for the formation of custom.
• Custom may be:
  • general;
  • regional;
  • local or particular.
• The persistent objector doctrine may allow a State that consistently objected during the formation of a customary rule to avoid its application in appropriate circumstances.
• The doctrine does not apply against jus cogens norms.
• Treaties may codify existing custom or contribute to the development of customary rules.
• A customary rule can bind a State even when that State is not a party to a treaty containing a similar rule.
• North Sea Continental Shelf Cases (1969) is a leading authority on State practice and opinio juris.
• Asylum Case (1950) is important for regional custom.
• Nicaragua v. United States (1986) demonstrates the existence of customary rules alongside treaty obligations.
• Customary international law allows international law to develop without a formal international legislature.
• The central formula to remember is:

General and consistent State practice + Opinio juris = Customary International Law

Conclusion

International custom is a dynamic and important source of Public International Law. It develops from the interaction between the actual conduct of States and their belief that such conduct reflects a legal obligation or entitlement. Article 38(1)(b) of the ICJ Statute provides its classical formulation, while cases such as North Sea Continental Shelf and Nicaragua v. United States demonstrate how international courts identify and apply customary rules. Unlike treaties, custom does not depend upon a single written agreement, allowing international law to develop gradually through State practice. At the same time, practice must be sufficiently general and consistent and accompanied by opinio juris before it can acquire customary legal status.