Corporate and Business Law (Global) · International trade, international legal regulation and conflict of laws

Sources of International Law and Legal Systems

Updated 11 October 2026 · Fact-checked

International law is the body of rules that governs relations between states and other international actors. Its main sources are treaties, customary international law and general principles of law, with judicial decisions and scholarly writing as subsidiary aids. Monism and dualism explain how those rules operate inside national legal systems.

Understand Sources of International Law and Legal Systems

International law is different from national law. No world parliament passes it, and no single world court enforces it. States create the rules themselves, mostly by agreeing to them. That is why the sources matter: they tell you where a binding rule comes from.

The usual list of sources follows Article 38 of the Statute of the International Court of Justice. There are three primary sources. A treaty is a written agreement between states (or international organisations) governed by international law. Customary international law arises from consistent state practice that states follow because they feel legally bound to do so. General principles of law are principles recognised by legal systems around the world, such as good faith and the idea that a wrong should be put right.

There are also subsidiary means for finding the law. These are judicial decisions and the writings of highly qualified legal scholars. They do not create law. They help show what the law is. International court decisions are not binding precedent in the way a common law court decision is.

Custom has two parts. State practice is the objective part: what states actually do, how widely and how consistently. Opinio juris is the subjective part: the belief that the practice is required by law. Habit or courtesy alone is not custom. Treaties generally bind only the states that are party to them. Custom can bind states generally, unless a state has consistently objected while the rule was forming.

Next, how does international law work inside a country? Monism treats international and national law as one system. International law can apply domestically without a separate step. Dualism treats them as two separate systems. International law applies domestically only if the state's own law adopts it, for example by passing a statute. Each country decides its own approach, so you should apply the approach the question gives you. Many states mix the two: custom may apply automatically while treaties need legislation.

Key formulas to remember

Primary sources of international law: Treaties + Customary international law + General principles of law; These are the three sources usually listed in Article 38 of the ICJ Statute. Learn them as a set.
Subsidiary means: Judicial decisions + Writings of qualified scholars; They help identify the law. They do not create it.
Customary international law: Custom = State practice + Opinio juris; Both elements are needed. Practice without a sense of legal obligation is only habit or courtesy.
Treaty binding effect: A treaty binds the states that are party to it; Generally a treaty does not bind non-parties. Parties are expected to perform in good faith (pacta sunt servanda).
Monism: International law + national law = one legal system; International law can be applied domestically without separate legislation.
Dualism: International law ≠ national law (two separate systems); International law applies domestically only if national law adopts it, often by legislation.

How to solve Sources of International Law and Legal Systems questions

Use this method for any question on sources of international law or how it fits with national law.

1Read the question and decide what it tests: identifying a source, explaining a source, or explaining the link to national law.
2If it asks for a source, name it exactly: treaty, custom, general principles, or a subsidiary means.
3Match the facts to the definition. A written agreement between states points to a treaty. Long, widespread state conduct points to custom.
4For custom, test both elements. Ask what states have done (state practice) and whether they act out of a sense of legal duty (opinio juris).
5For a treaty, ask who the parties are. Remember that it generally binds only parties.
6For national law questions, ask whether the system is monist or dualist. Check whether legislation is needed to bring the rule in.
7Apply the rule to the facts and write a clear conclusion in one sentence.
8For Section B, check each option against the exact definition before choosing. Wrong options often swap the terms.

Quickest way: Three-source and two-system check

When to use it: Use this in Section A or B objective questions when you have under two minutes per question.

Written agreement between states? Choose treaty.
Long-standing common conduct plus belief it is legally required? Choose custom.
Principle found across many legal systems, such as good faith? Choose general principles.
Court decisions or scholars' writings? Choose subsidiary means, not a creating source.
Needs a national statute before it applies? Dualism. Applies directly as part of one system? Monism.

Common mistakes in Sources of International Law and Legal Systems

Treating court decisions and scholarly writings as primary sources.
Students think of case law as the main source of law because of national systems.
Fix: Remember that they are subsidiary means. They help identify the law but do not create it.
Saying custom needs only state practice.
Students focus on what states do and forget the mental element.
Fix: Always state both parts: state practice and opinio juris.
Saying a treaty binds every state.
Students assume international law works like a statute that applies to everyone.
Fix: Say that a treaty generally binds only the states that are party to it.
Mixing up monism and dualism.
The two words look alike and are learned as labels without the idea behind them.
Fix: Link the prefix: mono means one system, dual means two systems. Dualism needs a bridge, usually legislation.
Assuming international court decisions are binding precedent like in common law courts.
Students carry over the doctrine of precedent from national law.
Fix: Treat them as persuasive evidence of the law, not binding precedent for later cases.

Worked examples

Example 1

Several states have for decades refused to prosecute foreign diplomats for ordinary offences, and they do so because they believe international law requires this. No treaty covers the point. Identify the source of the rule and explain why.

Show the solution

Step 1: Note that there is no treaty, so the source is not a treaty.
Step 2: The conduct has gone on for decades. This shows consistent state practice.
Step 3: The states act from a belief that the law requires it. This is opinio juris.
Step 4: Both elements are present, so the rule is customary international law.

Answer: The rule is customary international law, because it combines consistent state practice with opinio juris.

Example 2

State X has signed and ratified a treaty on environmental standards. State X follows dualism. A company in State X says it is not bound by the treaty because the national parliament has passed no law on it. Explain whether the company is correct.

Show the solution

Step 1: A treaty binds State X in international law as it has ratified it.
Step 2: Under dualism, international law and national law are separate systems.
Step 3: A dualist state needs national legislation to make a treaty apply within its own courts and to its citizens.
Step 4: No legislation has been passed, so the treaty does not directly impose duties on the company in State X's courts.
Step 5: State X remains in breach of its international obligation if it does not implement the treaty.

Answer: The company is correct in domestic law, because a dualist state needs legislation to give a treaty effect. State X is still bound on the international plane and may be in breach.

Exam tips

Learn the three primary sources as a list and be ready to name them in any order.
In custom questions, state both elements: state practice and opinio juris. Examiners reward the pair.
Read option wording carefully in objective questions. Distractors often call a subsidiary means a primary source.
For monism and dualism, give one line on each and say what happens to a treaty in a dualist state.
Link to other topics. Sources feed into how conflict of laws and trade regulation work in practice.

Practice questions from International trade, international legal regulation and conflict of laws

Sources of International Law and Legal Systems in other exams

The same ground in other exams, if you are preparing for more than one or want another angle on it.

Sources of International Law and Legal Systems: frequently asked questions

What are the main sources of international law?

The three main sources are treaties, customary international law and general principles of law. Judicial decisions and scholarly writings are subsidiary means that help identify the law. Learn both groups for the exam.

What is the difference between monism and dualism?

Monism sees international law and national law as one system, so international law can apply domestically directly. Dualism sees them as two separate systems, so international rules usually need national legislation first. Each state follows its own approach.

What is opinio juris?

Opinio juris is the belief by states that a practice is required by law. It is the mental element of custom. Together with consistent state practice it forms customary international law.

Does a treaty bind states that did not sign it?

Generally no. A treaty binds the states that are party to it. Some treaty rules may also reflect custom, and custom can bind states more widely.