Peace Palace Library

13/10/2026

Start Your Research: (2) Custom https://peacepalacelibrary.nl/start-your-research-2-custom A basic introduction to international legal research in the Peace Palace Library. Your road map for approaching an international legal research problem. Step 2.

13/10/2026

Start Your Research: (2) Custom
https://peacepalacelibrary.nl/start-your-research-2-custom

A basic introduction to international legal research in the Peace Palace Library. Your road map for approaching an international legal research problem. Step 2.

Article 38(1)(b) of the Statute of the International Court of Justice states that the Court shall apply 'international custom, as evidence of a general practice accepted as law' to settle international disputes. A rule of customary international law is demonstrated by showing evidence of two essential elements: 'general practice' and 'accepted as law'. The first element is objective and the second is subjective, referred to as opinio juris sive necessitates, or simply, opinio juris.

Customary international law develops in a more informal and decentralised manner than treaty law. It arises organically, based on the activities and beliefs of States acting on the international stage over time. It is unwritten and contains no formal enforcement mechanisms whatsoever. In the context of legal research, one must be focused on the resources that reveal patterns of behaviour as States pursue their national interests and on the motivations accompanying those patterns of behaviour. Once a rule of customary international law emerges, it binds all States, whether or not they participated in its development.

There is no single, clear path for finding and determining rules of customary international law because the documents vary depending on the subject matter of the custom. Using publications and documents from multiple foreign jurisdictions is particularly challenging. Taking the time to understand customary international law as a source of law before starting the research process is essential. By following the strategies discussed within the reference works suggested below, legal researchers should be able to handle the research challenges posed by customary international law as a source of law. Please, consult our Research Guide on Customary International Law as a starting point for finding the right resources.

You might consider our Library Instruction on how to conduct legal research in the Library at all levels.

References

Guide to International Legal Research, Fifth Edition, Newark, LexisNexis Matthew Bender, 2003.

Hoffman, M.B. and R.C. Berring Jr., International Legal Research in a Nutshell, Second Edition, St. Paul, West Academic Publishing, 2017.

Kuehl, H.F. and M.A. O'Brien, International Legal Research in a Global Community, Durham, Carolina Academic Press, 2018.

Winer, A.S., M.A.E. Archer, and L. Louis-Jacques, International Law Legal Research, Durham, Carolina Academic Press, 2013.

13/10/2026

Start Your Research: (2) Custom

A basic introduction to international legal research in the Peace Palace Library. Your road map for approaching an international legal research problem. Step 2.

https://peacepalacelibrary.nl/start-your-research-2-custom

Article 38(1)(b) of the Statute of the International Court of Justice states that the Court shall apply 'international custom, as evidence of a general practice accepted as law' to settle international disputes. A rule of customary international law is demonstrated by showing evidence of two essential elements: 'general practice' and 'accepted as law'. The first element is objective and the second is subjective, referred to as opinio juris sive necessitates, or simply, opinio juris.

Customary international law develops in a more informal and decentralised manner than treaty law. It arises organically, based on the activities and beliefs of States acting on the international stage over time. It is unwritten and contains no formal enforcement mechanisms whatsoever. In the context of legal research, one must be focused on the resources that reveal patterns of behaviour as States pursue their national interests and on the motivations accompanying those patterns of behaviour. Once a rule of customary international law emerges, it binds all States, whether or not they participated in its development.

There is no single, clear path for finding and determining rules of customary international law because the documents vary depending on the subject matter of the custom. Using publications and documents from multiple foreign jurisdictions is particularly challenging. Taking the time to understand customary international law as a source of law before starting the research process is essential. By following the strategies discussed within the reference works suggested below, legal researchers should be able to handle the research challenges posed by customary international law as a source of law. Please, consult our Research Guide on Customary International Law as a starting point for finding the right resources.

Step 1 ← Step 2 → Step 3

You might consider our Library Instruction on how to conduct legal research in the Library at all levels.

References
Guide to International Legal Research, Fifth Edition, Newark, LexisNexis Matthew Bender, 2003.
Hoffman, M.B. and R.C. Berring Jr., International Legal Research in a Nutshell, Second Edition, St. Paul, West Academic Publishing, 2017.
Kuehl, H.F. and M.A. O'Brien, International Legal Research in a Global Community, Durham, Carolina Academic Press, 2018.
Winer, A.S., M.A.E. Archer, and L. Louis-Jacques, International Law Legal Research, Durham, Carolina Academic Press, 2013.

09/10/2026

Research Guide
Treaties (updated)

https://peacepalacelibrary.nl/research-guide/treaties

Treaties play a fundamental role in international relations. They are recognized as a source of international law in Article 38 of the Statute of the International Court of Justice. Therefore, the Vienna Convention on the Law of Treaties was adopted in 1969. The Vienna Convention regulates the conclusion and entry into force of treaties. Moreover, it regulates the application and interpretation of treaties as well as the amendment, invalidity and termination of treaties.

According to Article 2 of this multilateral agreement, a 'treaty’ means an international agreement concluded between States in written form and governed by international law. Whether is is embodied in a single instrument or in two or more related instruments and whatever its particular designation. A treaty concluded between one or more States and one or more international organizations, or between international organizations, can also be referred to as a treaty. According to Article 38 of the Statute of the International Court of Justice, international conventions must be applied by the Court, when deciding disputes that are submitted to it. These conventions can be general or particular. They establish rules expressly recognized by the contesting states. From this principle, one can conclude that international conventions are one of the principal sources of public international law.

This Research Guide is intended as a starting point for research on the Law of Treaties. It provides the basic materials available in the Peace Palace Library, both in print and electronic format. Handbooks, leading articles, bibliographies, periodicals, serial publications and documents of interest are presented in the Selective Bibliography section. Links to the PPL Catalogue are inserted. The Library's subject headings (keywords) Treaties, and Law of Treaties are instrumental for searching through the Catalogue. Special attention is given to our subscriptions on databases, e-journals, e-books and other electronic resources. Finally, this Research Guide features links to relevant websites and other online resources of particular interest.

Research Guide intended as a starting point for research on Treaties. It provides the legal resources available in the Peace Palace Library.

02/10/2026

Research Guide
Public International Law (updated)

https://peacepalacelibrary.nl/research-guide/public-international-law

Public International Law refers to the body of legal rules and principles which determine the international rights and obligations of nation-states and regulate the operations of international organizations. Also non-governmental entities and individuals have become part of Public International Law. The scope of activities covered by Public International Law has grown over the years to include additional topics, e.g., human rights, international environmental law, international criminal law, and international economic law. The normative system of Public International Law is derived from the four sources, listed in Article 38.1 of the International Court of Justice Statute, i.e., treaties, customary international law, general principles of law, and ‘judicial decisions and the teachings of the most highly qualified publicists of the various nations, as a subsidiary means for the determination of rules of law’. Various international courts and arbitration tribunals provide for the settlement of disputes in the field of Public International Law. For detailed guidance on individual topics of Public International Law, see the individual research guides on the various special topics.

This Research Guide is intended as a starting point for research on Public International Law. It provides the basic legal materials available in the Peace Palace Library, both in print and electronic format. Handbooks, leading articles, bibliographies, periodicals, serial publications and documents of interest are presented in the Selective Bibliography section. Links to the PPL Catalogue are inserted. The Library's subject heading (keyword) Public International Law is instrumental for searching through the Catalogue. Special attention is given to our subscriptions on databases, e-journals, e-books and other electronic resources. Finally, this Research Guide features links to relevant websites and other online resources of particular interest.

25/09/2026

Research Guide
History of International Law (updated)

https://peacepalacelibrary.nl/research-guide/history-international-law

While the origins of the modern system of international law can be traced back 400 years, the development of the concepts and practises that would underpin that system can be traced back to ancient historical politics and relationships thousands of years old. Important concepts are derived from the practice between Greek city-states and the Roman law concept of ius gentium, which regulated contacts between Roman citizens and non-Roman people. These principles were not universal however. In East Asia, political theory was based not on the equality of states, but rather the cosmological supremacy of the sovereign, that is the emperor of China.

The Peace Palace Library has an impressive old and rare books collection of 10.000 titles printed before 1850. Since 1913 the Library has acquired a historical legal collection representing the various periods of thinking about international law, dominated by the Spanish, French or English schools. Writers from the 16th century: Jean Bodin, Ayala, Vitoria, the 17th century: Hugo Grotius, Gentili, Pufendorf, Zouche, and 18th century: Bynkershoek, Wolff, Martens, Vattel, and 19th century Bluntschli, to name only a few. In total about 2400 authors represent the origins of international law. Modern authors on the history of international refer to these sources for their research.

This Research Guide is intended as a starting point for research in the field of the History of International Law. It provides the basic materials available in the Peace Palace Library, both in print and electronic format. Handbooks, leading articles, bibliographies, periodicals, serial publications and documents of interest are presented in the Selective Bibliography section. Links to the PPL Catalogue are inserted. The Library's subject heading (keyword) History of International Law is instrumental for searching through the Catalogue. Special attention is given to our subscriptions on databases, e-journals, e-books and other electronic resources. Finally, this Research Guide features links to relevant websites and other online resources of particular interest.

Research Guide intended as a starting point for research on the History of International Law. It provides the legal resources available in the Library.

23/09/2026

Start Your Research: (1) Treaties

https://peacepalacelibrary.nl/start-your-research-1-treaties

A basic introduction to international legal research in the Peace Palace Library. Your road map for approaching an international legal research problem. Step 1.

Treaty research is generally carried out first when handling an international legal research project. Article 38(1)(a) of the Statute of the International Court of Justice states that the Court shall apply 'international conventions, whether general or particular, establishing rules expressly recognized by the contesting states' as a source of law in reaching decisions. The phrase 'international conventions' is generally interpreted broadly. It is meant to include not only international conventions but all other designations, such as the word treaty that - according to the Vienna Convention on the Law of Treaties (1969) - is used for 'an international agreement concluded between States in written form and governed by international law'. Other examples are protocol, accord, act, charter, or covenant, among other instruments. While there are subtle distinctions among these designations, they may fairly be lumped together for purposes of conducting treaty research.

For conducting thorough treaty research it is important to have a solid grasp of the treaty-making process and to have an understanding of where and how to find collections of treaty documents. It is advisable to collect as much information as possible related to any particular treaty. So find the authoritative text, identify the depositary, and check the status of the treaty: identify signatories and parties to the treaty, determine the signature, ratification, acceptance, approval, accession and entry into force dates, find and read the reservations, understandings and declarations, etc.

Treaty Collections

There are different types of treaty collections that will help you to decide where to begin your treaty research project. Treaty collections may be differentiated based on their scope and coverage. They may be general, historical, national, regional, or restricted to a particular subject area. A combination of these types of treaty collections also exists. Some examples:

United Nations Treaty Series (1946- current) [print] [HeinOnline]
League of Nations Treaty Series (1920-1946) [print] [HeinOnline]
Consolidated Treaty Series (1648-1920)
United States Treaties and Other International Agreements (1950- current)
European Treaty Series (1949-2004)
Council of Europe Treaty Series (2004- current)

The Peace Palace Library Catalogue is your gateway to the various treaty collections. We offer access to all types of collections - electronically or in print, and cover all topics of international law. The Peace Palace Library Research Guides, prepared and maintained by our team of Law Librarians, are another powerful tool to find treaty collections. The Research Guides suggest certain collections, treaty compilations, databases and other materials considered indispensable for doing treaty law research on your specific topic.

You might consider our Library Instruction on how to conduct legal research in the Library at all levels.

References

Guide to International Legal Research, Fifth Edition, Newark, LexisNexis Matthew Bender, 2003.

Hoffman, M.B. and R.C. Berring Jr., International Legal Research in a Nutshell, Second Edition, St. Paul, West Academic Publishing, 2017.

Kuehl, H.F. and M.A. O'Brien, International Legal Research in a Global Community, Durham, Carolina Academic Press, 2018.

Winer, A.S., M.A.E. Archer, and L. Louis-Jacques, International Law Legal Research, Durham, Carolina Academic Press, 2013.

18/09/2026

Research Guide
Customary International Law (updated 20 September 2021)

peacepalacelibrary.nl/research-guide/customary-international-law

According to Article 38 of its Statute, the International Court of Justice 'whose function is to decide in accordance with international law such disputes as are submitted to it,' has to apply, inter alia, 'international custom.' This source of public international law is described, in the same Article, as 'evidence of a general practice accepted as law.' This description of international custom, even though it has been criticized for its exact formulation, at least makes clear that international custom generally refers to a description of State practice. But only when such practice is accepted by the States themselves as legally required, it is considered customary law. Once a certain practice is understood to be customary law, States are obliged to act as the rule of customary international law prescribes.

International customary law is probably the most disputed and discussed source of international law. For example, it is not clear when a particular State practice becomes a legally binding State practice. It is also unclear how one can identify a rule of international custom, or how one can prove its existence. Important elements are the manifestations of State practice, like declarations, statements or other official documents.

The International Law Commission appointed Sir Micheal Wood as Special Rapporteur concerning the issue of Customary International Law. His reports and recommendations are available in the bibliography under documents.

This Research Guide is intended as a starting point for research in the field of Customary International Law. It provides the basic materials available in the Peace Palace Library, both in print and electronic format. Handbooks, leading articles, bibliographies, periodicals, serial publications and documents of interest are presented in the Selective Bibliography section. Links to the PPL Catalogue are inserted. The Library's subject heading (keyword) International Customary Law is instrumental for searching through the Catalogue. Special attention is given to our subscriptions on databases, e-journals, e-books and other electronic resources. Finally, this Research Guide features links to relevant websites and other online resources of particular interest.

Research Guide intended as a starting point for research on International Customary Law. It provides the legal resources available in the Library.

15/09/2026

We look forward to welcoming the 45th Annual Course of the International Association of Law Libraries to the Netherlands in 2027!

Together with the Sanders Law Library, Erasmus School of Law, Rotterdam, we will explore how law, legal information, and international cooperation help us navigate both Dutch and international waters.

The Annual Course will be held 26-30 September 2027 in The Hague and Rotterdam.

14/09/2026

Start Your Research: (1) Treaties https://peacepalacelibrary.nl/start-your-research-1-treaties A basic introduction to international legal research in the Peace Palace Library. Your road map for approaching an international legal research problem. Step 1.