1.  Art. 38 (1) (b) ICJ Statute

6  Article 38 (1) Statute of the International Court of Justice (‘ICJ Statute’), which is widely considered to set out the sources of international law, lists in its sub-para. (b) ‘international custom, as evidence of a general practice accepted as law’. The fact that the mention of treaties (in sub-para. (a)) precedes that of customary international law has no implication as to the hierarchy between the rules belonging to these sources. It simply reflects the idea that, when a concrete case is submitted to a court, treaties, as special rules applicable between the parties, are to be considered and applied before customary rules, which are in principle more general.

7  It is often remarked that Art. 38 (1) (b) ICJ Statute is not a model of drafting. It might have been clearer had it referred to ‘international custom as evidenced by a general practice accepted as law’. This is how the provision is generally read, making the relationship between the rule and its constituent elements seem more logical. In any case, it seems sufficiently clear from Art. 38 (1) (b) ICJ Statute that two elements are required: a general practice, and the acceptance of this practice as law. The existence of each of these elements in any particular case is a matter for appreciation in all the circumstances.

2.  The Two Constituent Elements of Customary International Law

8  The judgments of the Permanent Court of International Justice (PCIJ) and the ICJ have been constant in stating that the existence of a rule of customary international law requires the presence of two elements: a general practice, and its acceptance as law. Already in 1927, in the case of The ‘Lotus’ (France v Turkey), the PCIJ stated that international law is based on the will of States expressed in conventions or in ‘usages generally accepted as expressing principles of law’ (at 18). The Court moreover stressed the need for both elements to be present in order to reach a finding of customary international law (at 28, Lotus, The).

9  The ICJ has thrown further light on the two constituent elements of customary international law, especially in the North Sea Continental Shelf Cases (1969), where it stated that for a customary rule to exist the acts concerned must not only

Similarly, in the Military and Paramilitary Activities in and against Nicaragua Case (Nicaragua v United States of America) (1986) (‘Nicaragua Case’), the Court stated that ‘for a new customary rule to be formed, not only must the acts concerned “amount to a settled practice” but they must be accompanied by the opinio juris sive necessitatis’ (para. 207). In Delimitation of the Maritime Boundary in the Gulf of Maine Area (Canada/United States of America) (1984), a Chamber of the Court spoke of ‘customary rules whose presence in the opinio juris of States can be tested by induction based on the analysis of a sufficiently extensive and convincing practice, and not by deduction from preconceived ideas’ (para. 111) (Gulf of Maine Case). The same approach was adopted in the case concerning Jurisdictional Immunities of the State (Germany v Italy: Greece Intervening), where the Court stated that to determine the existence of a customary rule

Subsequent decisions of the Court, including Legal Consequences of the Separation of the Chagos Archipelago from Mauritius in 1965 (Chagos [Advisory Opinion]) and Question of the Delimitation of the Continental Shelf between Nicaragua and Colombia beyond 200 Nautical Miles from the Nicaraguan Coast (Nicaragua v Colombia) (‘Delimitation of the Continental Shelf beyond 200 Miles’), have followed suit.

10  That both elements must always be present for a customary rule to exist is further evident in the work of the International Law Commission (ILC), including its 2018 Draft Conclusions on Identification of Customary International Law. Conclusion 2 states that: ‘To determine the existence and content of a rule of customary international law, it is necessary to ascertain whether there is a general practice that is accepted as law (opinio juris)’. The accompanying commentary explains that ‘[p]ractice without acceptance as law (opinio juris), even if widespread and consistent, can be no more than a non-binding usage, while a belief that something is (or ought to be) the law unsupported by practice is mere aspiration’ (para. 4). Thus the two-element approach ‘serves to ensure that the exercise of identifying rules of customary international law results in determining only such rules as actually exist’ (ibid para. 1). By resolution 73/203, the United Nations General Assembly brought the Conclusions ‘to the attention of States and all who may be called upon to identify rules of customary international law, and encourage[d] their widest possible dissemination’ (para. 4).

11  The two-element approach has also been adopted in the practice of other international courts and tribunals, which have shown substantial reliance on the approach and case law of ICJ. The International Criminal Tribunal for the Former Yugoslavia (ICTY), for example, insisted that ‘to hold that a principle was part of customary international law, it has to be satisfied that State practice recognized the principle on the basis of supporting opinio juris’ (Prosecutor v Hadžihasanović [2003] para. 12). Internationalized (‘hybrid’), regional, arbitral, and domestic courts and tribunals also generally seek to follow this methodology.

3.  A General Practice

12  Practice is what the subjects of international law do and say. It is primarily the practice of States that is relevant in this regard, as the ILC explained in 2018: ‘[b]eing the primary subjects of the international legal system and possessing a general competence, States play a preeminent role in the formation of customary international law, and it is principally their practice that has to be examined in identifying it’ (para. 2 commentary to Conclusion 4). Such practice will often be attributable to individual States but may well be undertaken by States jointly.

13  In certain cases, the practice of international (intergovernmental) organizations in fulfilment of their respective mandates may also contribute to the creation, or expression, of rules of customary international law (see ILC’s Conclusion 4, para. 2, and accompanying commentary). Ascertainment and assessment of such participation in the customary process and of its relevance must be made with particular caution: first, because of the limited competence of the organizations, and, secondly, because it may be preferable to consider many manifestations of such practice, such as resolutions of the UN General Assembly, as practice of the States involved rather than of the organizations. Practice on issues that are directly relevant to the participation of international organizations in international relations will be particularly significant. This applies, for example, to the law of treaties concluded with or between international organizations (International Organizations or Institutions, External Relations and Cooperation), the international responsibility of international organizations (International Organizations or Institutions, Responsibility and Liability), or the law of succession between international organizations (International Organizations or Institutions, Succession). Relevant practice may also be found when, as in the case of the European Union, the international organization replaces, in whole or in part, its Member States in international relations, so that the practice of the organization may be equated with the practice of those States.

14  State Practice consists of conduct of the State, whether in the exercise of its executive, legislative, judicial, or other functions (see the ILC’s Conclusion 5 and commentary). It includes both physical and verbal acts as well as deliberate inaction, and may take a wide range of forms. The illustrative list offered by the ILC in its Conclusion 6 refers to

One example of such practice is the Freedom of Navigation Program pursued by the United States, inter alia through operational deployment of ships and aircraft, ‘to assert its navigation and overflight rights’, and to contest what it deems to be ‘excessive’ maritime claims of other States (Statement of R Reagan [1993] 1 Cumulative Digest of US Practice in International Law 1743). Other examples include abstaining from instituting criminal proceedings against foreign State officials (by reason of an immunity accorded to them under international law) and economic and other measures taken as countermeasures. All such conduct is directed at, or available to, other States; consequently, they can directly influence the attitude of other States or cause reactions on their part.

15  Certain forms of practice may be of particular significance in regard to the rule of customary international law in question. For example, in the case concerning Jurisdictional Immunities of the State, the ICJ explained that

16  In assessing the relevance and weight of specific instances of practice, the position of the State organ concerned is not unimportant. Statements by heads of State[s] or of government or by ministers of foreign affairs or of justice, for instance, may be recognized as carrying particular weight (Heads of Governments and Other Senior Officials). Statements of delegates to technical conferences may be seen as carrying less weight than those of legal experts to conferences with a high legal content. Greater weight will likely attach to decisions of higher (as opposed to lower) courts; decisions that have been overruled on the particular point will generally not be relevant. The practice of any particular State will in any event need to be assessed as a whole so as to determine accurately the position of the State on the matter in question (see the ILC’s Conclusion 7).

17  In order to contribute to the formation and identification of rules of customary international law, practice must be known to other States, so as to afford them an opportunity to respond to it. Unpublished practice, while it may have an impact on the incident it concerns, has reduced influence on the customary process as it remains unknown to most States. It may help in illuminating the attitude of the States concerned once it is made known, for instance when materials from archives are utilized before international courts and tribunals. Reluctance to make available practice by certain States, both large and small, and selectivity as to the documents made available, may reflect a political choice between the desire to avoid criticism and to make it easier to contradict previous practice, on the one hand, and the desire to exercise leadership and influence the customary process, on the other.

18  In addition to the systematic collections (digests) of State practice published in a significant number of States officially or by scholarly institutions, mention should also be made of the very useful collections of treaties (such as the United Nations Treaty Series) and of the judgments and often of the written and oral pleadings of international courts and tribunals. Documents of international organizations are published and made easily available to States. Of particular importance is the publication, by the United Nations (UN) and other international organizations, of the documents concerning the process of codification of international law, including studies of practice prepared in connection with the process and views of States obtained on the various drafts. In 2018, the UN Secretariat published a memorandum on ways and means for making the evidence of customary international law more readily available (UN ILC ‘Identification of Customary International Law: Ways and Means for Making the Evidence of Customary International Law More Readily Available: Memorandum by the Secretariat’).

19  The conduct of actors other than States and public international organizations (ie that of political movements, scholars, non-State armed groups, religious groups, etc; Non-Governmental Organizations) cannot be considered as relevant practice as such directly partaking in the formation of customary international law. The perception of these actors of what is permitted and of what is prohibited under customary international law, and their impact on public opinion, may nevertheless influence the perception of governments and ultimately their opinio iuris. The ILC recognized this in para. 3 of its Conclusion 4: ‘Conduct of other actors is not practice that contributes to the formation, or expression, of rules of customary international law, but may be relevant when assessing the practice [of States and, in certain cases, of international organizations]’. Particular mention was made in this respect of the International Committee of the Red Cross (ICRC), which plays an important role in shaping the practice of States concerning international humanitarian law (Humanitarian Law, International).

20  That is not to say that rules of customary international law are not applicable in respect of non-State actors. The rule on self-defence has been invoked against non-State armed groups (Terrorism), and rules of international humanitarian law have been considered applicable to irregular armed formations.

21  General practice as a constituent element of customary international law is sometimes referred to as the ‘material’ or ‘objective’ element (acceptance as law being the ‘subjective’ or ‘psychological’ element). The ILC captured the essence of this requirement in explaining that ‘[t]he relevant practice must be general, meaning that it must be sufficiently widespread and representative, as well as consistent’ (para. 1 Conclusion 8). The ICJ in the North Sea Continental Shelf Cases considered that the practice must be ‘both extensive and virtually uniform’ (para. 74).

22  The practice relevant for establishing the existence of a rule of customary international law need not be universal. At the same time, in determining whether a general practice exists, ‘an indispensable factor to be taken into account is the extent to which those States that are particularly involved in the relevant activity or are most likely to be concerned with the alleged rule (“specially affected States”) have participated in the practice’ (see para. 4 commentary to the ILC’s Conclusion 8; North Sea Continental Shelf Cases para. 74). While the expression ‘specially affected States’ has sometimes been questioned, it would, for instance, be difficult to determine the existence of a customary rule concerning the law of the sea in the absence of corresponding practice of the main maritime powers, or of the main coastal States, or, as the case may be, of the main fishing States, while the silence of less involved States would not be an obstacle to such determination. Similarly, rules on economic relations, such as those on foreign investment, require practice of the main investor States as well as that of the main States in which investment is made.

23  As regards the need for uniformity, the ICJ stated in the Nicaragua Case that ‘it does not consider that, for a rule to be established as customary, the corresponding practice must be in absolutely rigorous conformity with the rule’ (para. 186). General consistency with the rule is sufficient, if conduct inconsistent with it is treated as a breach and not as an indication of a new rule. Invoking exceptions or justifications for conduct that appears inconsistent with the rule, whether or not they are well-founded, ‘confirm[s] rather than weaken[s] the rule’ (ibid).

24  Notwithstanding that a general practice need not include the practice of all States, existing rules of customary international law are in principle binding on every State, whether or not it had taken part in the practice giving rise to those rules, including a State that came into existence after the rule in question arose (see, for example, Frontier Dispute [1986] para. 21; Frontier Dispute Case [Burkina Faso/Republic of Mali]). In fact, States comply with rules of customary international law even if, at the beginning of their existence as States, they held a contrary view, arguing that they had not been involved in the formation of customary international law. Such States have, nonetheless, developed positions and attitudes aiming at, and sometimes succeeding in, modifying certain existing rules and establishing others. Their participation in major endeavours to codify international law has also helped to develop their practice and involvement in the customary process.

25  Duration of the relevant practice over a long period of time has traditionally been considered as a requirement for the formation of rules of customary international law. More recent developments show, however, that customary rules may come into existence rapidly. This can be due to the urgency of dealing with new developments of technology, for instance, drilling technology as regards the rules on the continental shelf, or space technology as regards the rule on the freedom of extra-atmospheric space (Space Law). Or it may be due to the urgency of coping with widespread sentiments of moral outrage regarding crimes committed in conflicts such as those in Rwanda and Yugoslavia (see International Criminal Tribunal for Rwanda [ICTR]; International Criminal Tribunal for the Former Yugoslavia [ICTY]; Yugoslavia, Cases and Proceedings before the ICJ) that brought about the rapid formation of a set of customary rules concerning crimes committed in internal conflicts—Condorelli (1991) speaks of ‘coutume grande vitesse’, high-speed custom.

26  The intensification of practice within international organizations and conferences (see also International Law, Development through International Organizations, Policies and Practice), the adoption of multilateral treaties, and the existence and activity of specialized international tribunals have likewise contributed to the acceleration of the formation of rules of customary international law in these and other fields. Thus the ICJ in the North Sea Continental Shelf judgment considered that the passage of a considerable period of time was not mandatory. The Court stated in this regard that ‘the passage of only a short period of time is not necessarily, or of itself, a bar to the formation of a new rule of customary international law on the basis of what was originally a purely conventional rule’, provided that, in the period in question, short as it may be, State practice must be ‘both extensive and virtually uniform in the sense of the provision invoked’ (para. 74). The ILC similarly stated that ‘[p]rovided that the practice is general, no particular duration is required’ (para. 2 Conclusion 8).

27  Persistent objection to the formation of a rule of customary international law may have the effect of excluding the objecting State from the scope of a customary rule that has nonetheless come into existence. The ILC set out this persistent objector rule in Conclusion 15, according to which ‘[w]here a State has objected to a rule of customary international law while that rule was in the process of formation, the rule is not opposable to the State concerned for so long as it maintains its objection’ (para. 1). The Commission spelt out that ‘[t]he objection must be clearly expressed, made known to other States, and maintained persistently’ (para. 2). The status of persistent objector cannot be maintained in respect of customary rules belonging to the corpus of ius cogens.

28  While rules of customary international law normally apply to all States (other than a persistent objector), there are also rules of particular (as opposed to general) customary international law, which have arisen in the relations between certain States only and are applicable solely in the relations between them (Regional International Law). These may develop between as few as two States (Right of Passage over Indian Territory [Portugal v India] [Merits] [1960] 39; Right of Passage over Indian Territory Case). Moreover, as the ILC explained,

In determining the existence of a rule of particular customary international law, supporting practice and/or acceptance as law of all these States must be ascertained. The ICJ has pointed to a burden of proof in this regard, stating in the Asylum Case (Colombia/Peru) (1950) and subsequently that ‘[t]he Party which relies on a custom of this kind must prove that this custom is established in such a manner that it has become binding on the other Party’ (at 276; see also para. 2 of the ILC’s Conclusion 16).

4.  Acceptance as Law (opinio iuris)

29  Writers have long sought to capture the essence of the second constituent element of customary international law, some associating it with the will of States (that a customary rule shall come into being) and others considering it to be a belief (that a customary rule exists). Academic controversies have not stood in the way of establishing in any given case the existence (or otherwise) of this element, which requires that there be among States an acceptance as law of the practice in question. The ICJ has over the years used a range of expressions, including (in the North Sea Continental Shelf Cases) ‘a general recognition that a rule of law or legal obligation is involved’, ‘opinio juris’, ‘a belief that [the] practice is rendered obligatory by the existence of a rule of law requiring it’, and a ‘feel[ing] … [of] conforming to what amounts to a legal obligation’ (paras 74, 77). In the Chagos advisory opinion the Court spoke of ‘opinio juris, i.e. the acceptance of that practice as law (Article 38 of the Statute of the Court)’ (para. 142); in Delimitation of the Continental Shelf beyond 200 Miles it referred, inter alia, to ‘a sense of legal obligation’ (para. 77). The ILC, for its part, stated in Conclusion 9 that ‘[t]he requirement, as a constituent element of customary international law, that the general practice be accepted as law (opinio juris) means that the practice in question must be undertaken with a sense of legal right or obligation’. The Commission further explained that ‘[a] general practice that is accepted as law (opinio juris) is to be distinguished from mere usage or habit’ (ibid).

30  Acceptance as law (opinio iuris) is to be distinguished from other motives for action, such as comity, political expediency, or convenience, even if such considerations may sometimes also motivate the practice (see Delimitation of the Continental Shelf beyond 200 Miles para. 77). Instances of practice that go against the interest of the State from which they come, or that entail for them significant costs in political, military, economic, or other terms, may be significant in that they are less likely to reflect reasons other than legal obligation.

31  Opinio iuris is to be sought with respect to both the States engaging in the relevant practice and those in a position to react to it (see Nicaragua Case para. 207). It is not necessary to establish that all States have accepted the practice as law; ‘it is broad and representative acceptance, together with no or little objection, that is required’ (para. 5 commentary to ILC’s Conclusion 9).

32  Evidence of opinio iuris may take a wide range of forms: the ILC listed, in a non-exhaustive manner, ‘public statements made on behalf of States; official publications; government legal opinions; diplomatic correspondence; decisions of national courts; treaty provisions; and conduct in connection with resolutions adopted by an international organization or at an intergovernmental conference’, as well as (under certain conditions) failure to react to a certain practice (see Conclusion 10). As the ICJ made clear in Delimitation of the Continental Shelf beyond 200 Miles, in certain circumstances the general practice may itself be seen as an expression of opinio iuris (para. 77). In this regard it is necessary to exercise particular caution, lest the two constituent elements of customary international law collapse into one.

33  The increase in the number and frequency of multilateral forums, such as the UN General Assembly and its Sixth Committee (United Nations, General Assembly; United Nations, Sixth Committee), codification conferences, etc, where States meet to develop or discuss international law, gives States many more occasions than in the past to express views as to customary international law. The same may be said of the growing recourse to international legal proceedings. This has increased the quantity of what States say, even though it has also made it more difficult sometimes to distinguish whether what they say is what they believe is customary international law or what, in the light of strategies developed in their foreign legal policy, they may want to become customary international law.

34  States may express their opinio iuris jointly. For example, in 1989 the US and the Soviet Union issued a joint statement underscoring that the provisions of the United Nations Convention on the Law of the Sea of 1982 ‘in respect to traditional uses of the sea, generally constitute international law and practice and balance fairly the interests of all States’, and submitted to all States their ‘uniform interpretation’ of these rules (Union of Soviet Socialist Republics–United States: Joint Statement with Attached Uniform Interpretation of Rules of International Law Governing Innocent Passage [1989] 28 ILM 1444). Joint or coordinated positions having a bearing on customary international law are often issued by the Member States of the European Union (see also European Union Law and International Law). Declarations and statements made, or documents submitted, on behalf of groups of States (such as the Group of 77 [G77], the European Union and its Member States, etc), in the framework of international organizations, or conferences, may also contribute to the density of State practice and to its rapid development.

35  Certain resolutions of the UN General Assembly, such as those setting out declarations of principles, may be considered as relevant, especially when adopted without a vote, in order to ascertain the opinio iuris of the States adopting them. In assessing their significance, caution is particularly necessary, as States often participate in the adoption of these resolutions in view of the fact that they are not binding. Declarations made by States upon adoption of a resolution may also give indications relevant for assessing their significance. It is also necessary to distinguish provisions that can be considered as evidence of opinio iuris from those expressing the will to introduce new rules.

36  In the Nicaragua Case, the ICJ stated that ‘opinio juris may, though with all due caution, be deduced from, inter alia, the attitude of the Parties and the attitude of States towards certain UNGA resolutions’ (para. 188). In Legality of the Threat or Use of Nuclear Weapons (Advisory Opinion) (1996), the ICJ stated that these resolutions ‘can, in certain circumstances, provide evidence important for establishing the existence of a rule or the emergence of an opinio juris’ (para. 70; Nuclear Weapons Advisory Opinions). The relevant considerations for each resolution are ‘its content and the conditions of its adoption’ (ibid). Moreover, ‘a series of resolutions may show the gradual evolution of the opinio juris required for the establishment of a new rule’ (ibid). In a number of more recent cases the ICJ supported its statement that certain rules belong to the corpus of customary international law by relying on one or more UN General Assembly resolutions, eg in the judgment on Armed Activities in the Territory of the Congo (Democratic Republic of the Congo v Uganda) (2005) (para. 244) as regards permanent sovereignty over natural resources (Natural Resources, Permanent Sovereignty over), and in the Chagos advisory opinion (paras 150–56). The Iran-United States Claims Tribunal synthesized the view prevalent in international arbitration tribunals as follows:

The ILC’s Conclusion 12 is to similar effect.

Cited Bibliography

• L Condorelli ‘La coutume’ in M Bedjaoui (ed) Droit international: Bilan et perspectives (Pedone Paris 1991) 187–221.

Further Bibliography

• H Waldock ‘General Course on Public International Law’ (1962) 106 RdC 39–53.

• RR Baxter ‘Treaties and Custom’ (1970) 129 RdC 27–105.

• R-J Dupuy ‘Coutume sage et coutume sauvage’ in G Abi-Saab (ed) Mélanges offerts à Charles Rousseau: La Communauté Internationale (Pedone Paris 1974) 75–87.

• G Abi-Saab ‘La coutume dans tous ses états ou le dilemme du développement du droit international général dans un monde éclaté’ in Istituto di Diritto Internazionale e della Navigazione Genova and others (eds) Le droit international à l’heure de sa codification: Etudes en honneur de Roberto Ago vol 1 (Giuffré Milano 1987) 53–65.

• A Cassese and JHH Weiler (eds) Change and Stability in International Law-Making (de Gruyter Berlin 1988).

• K Wolfke Custom in Present International Law (2nd revised edn Kluwer Dordrecht 1993).

• ME Villiger Customary International Law and Treaties: A Manual on the Theory and Practice of the Interrelation of Sources (2nd edn Kluwer The Hague 1997).

• B Cheng ‘Some Remarks on the Constituent Element(s) of General (or So-called Customary) International Law’ in A Anghie and G Sturgess (eds) Legal Visions of the 21st Century: Essays in Honour of Judge Christopher Weeramantry (Kluwer The Hague 1998) 377–90.

• OA Elias and CL Lim The Paradox of Consensualism in International Law (Kluwer The Hague 1998).

• MH Mendelson ‘The Formation of Customary International Law’ (1998) 272 RdC 155–410.

• M Byers Custom, Power and the Power of Rules: International Relations and Customary International Law (CUP Cambridge 1999).

• AE Roberts ‘Traditional and Modern Approaches to Customary International Law: A Reconciliation’ (2001) 95(4) AJIL 757–91.

• GP Buzzini ‘La théorie des sources face au droit international general: Réflexions sur l’émergence du droit objectif dans l’ordre juridique international’ (2002) 106 RGDIP 581–617.

• J Crawford and T Viles ‘International Law on a Given Day’ in J Crawford International Law as an Open System: Selected Essays (Cameron May London 2002) 69–94.

• Société Française pour le droit international (ed) La pratique et le droit international: Colloque de Genève (Pedone Paris 2004).

• J-M Henckaerts ‘Study on Customary International Humanitarian Law: A Contribution to the Understanding and Respect for the Rule of Law in Armed Conflict’ (2005) 87 IRRC 175–212.

• T Meron ‘Revival of Customary Humanitarian Law’ (2005) 99(4) AJIL 817–34.

• P Tomka ‘Custom and the International Court of Justice’ (2013) 12 Law and Practice of International Courts and Tribunals 195–216.

• S Talmon ‘Determining Customary International Law: The ICJ’s Methodology between Induction, Deduction and Assertion’ (2015) 26(2) EJIL 417–43.

• O Sender and M Wood ‘The International Court of Justice and Customary International Law: A Reply to Stefan Talmon’ (30 November 2015) EJIL: Talk! <https://www.ejiltalk.org/the-international-court-of-justice-and-customary-international-law-a-reply-to-stefan-talmon/>.

• CA Bradley (ed) Custom’s Future: International Law in a Changing World (CUP Cambridge 2016).

• L Lijnzaad and Council of Europe (eds) The Judge and International Custom (Brill Nijhoff Leiden 2016).

• BD Lepard (ed) Reexamining Customary International Law (CUP Cambridge 2017).

• A Pellet and D Müller ‘Article 38’ in A Zimmermann and others (eds) The Statute of the International Court of Justice: A Commentary (3rd edn OUP Oxford 2019) 819–962.

• H Thirlway The Sources of International Law (2nd edn OUP Oxford 2019) 60–105.

• T Treves ‘The Expansion of International Law: General Course on Public International Law’ (2019) 398 RdC 137–218.

• M Wood ‘Customary International Law and the General Principles of Law Recognized by Civilized Nations’ (2019) 21 International Community Law Review 307–24.

• J d’Aspremont The Discourse on Customary International Law (OUP Oxford 2021).

• P-M Dupuy (ed) Customary International Law (Elgar Cheltenham 2021) (a collection of articles, mainly from legal journals, with an introduction).

• F Iurlaro The Invention of Custom: Natural Law and the Law of Nations, ca. 1550-1750 (OUP Oxford 2021).

• MN Shaw International Law (9th edn CUP Cambridge 2021) 60–78.

• M Forteau A Pellet and A Miron Droit international public (9th edn LGDJ Paris 2022) 387–423.

• V Lanovoy ‘Customary International Law in the Reasoning of International Courts and Tribunals’ in P Merkouris J Kammerhofer and N Arajärvi (eds) The Theory, Practice, and Interpretation of Customary International Law (CUP Cambridge 2022) 231–55.

• O Sender and M Wood Identification of Customary International Law (OUP Oxford 2024).

Cited Documents

• Rome Statute of the International Criminal Court (adopted 17 July 1998, entered into force 1 July 2002) 2187 UNTS 90.

• Statute of the International Court of Justice (adopted 26 June 1945, entered into force 24 October 1945) 145 BSP 832.

• UN ILC ‘Draft Conclusions on Identification of Customary International Law’ [2018] UNYBILC vol II part II 90–113.

• UN ILC ‘General Principles of Law: Text of the Draft Conclusions Provisionally Adopted by the Drafting Committee on First Reading’ (12 May 2023) UN Doc A/CN.4/L.982.

• UN ILC ‘Identification of Customary International Law: Ways and Means for Making the Evidence of Customary International Law More Readily Available: Memorandum by the Secretariat’ (2018) UN Doc A/CN.4/710.Rev1.

• UNGA Res 56/83 ‘Responsibility of States for Internationally Wrongful Acts’ (12 December 2001) UN Doc A/RES/56/83, Annex.

• UNGA Res 73/203 ‘Identification of Customary International Law’ (20 December 2018) UN Doc A/RES/73/203.

• Vienna Convention on Diplomatic Relations (done 18 April 1961, entered into force 24 April 1961) 500 UNTS 95.

• Vienna Convention on the Law of Treaties (concluded 23 May 1969, entered into force 27 January 1980) 1155 UNTS 331.

• Vienna Convention on the Law of Treaties between States and International Organizations or between International Organizations (adopted 20 March 1986, not yet entered into force) (1986) 25 ILM 543.

Further Documents

• ILA ‘Resolution No 16/2000: Formation of General Customary International Law’ (2000) 69 ILA Conference Report.

Cited Cases

• Affaire concernant le filetage á l’intérieur du golfe du Saint-Laurent entre le Canada et la France [1986] 90 RGDIP 713.

• Ahmadou Sadio Diallo (Republic of Guinea v Democratic Republic of the Congo (Preliminary Objections) [2007] ICJ Rep 582.

• Amoco International Finance Corporation v Islamic Republic of Iran (14 July 1987) (1988) 15 Iran-US CTR 189.

• Armed Activities on the Territory of the Congo (Democratic Republic of the Congo v Uganda) [2005] ICJ Rep 168.

• Asylum Case (Colombia/Peru) [1950] ICJ Rep 266.

• Continental Shelf (Libyan Arab Jamahiriya/Malta) (Merits) [1985] ICJ Rep 13.

• Continental Shelf (Tunisia/Libyan Arab Jamahiriya) [1982] ICJ Rep 18.

• Delimitation of the Continental Shelf between the United Kingdom of Great Britain and Northern Ireland, and the French Republic (UK v France) (30 June 1977) 18 RIAA 3.

• Delimitation of the Maritime Boundary in the Gulf of Maine Area (Canada/United States of America) [1984] ICJ Rep 246.

• Frontier Dispute (Burkina Faso/Republic of Mali) (Judgment) [1986] ICJ Rep 554.

• Jurisdictional Immunities of the State (Germany v Italy: Greece Intervening) [2012] ICJ Rep 99.

• Legal Consequences of the Separation of the Chagos Archipelago from Mauritius in 1965 (Advisory Opinion) [2019] ICJ Rep 95.

• Legality of the Threat or Use of Nuclear Weapons (Advisory Opinion) [1996] ICJ Rep 226.

• The ‘Lotus’ (France v Turkey) (Merits) (7 September 1927) PCIJ Rep Series A No 10.

• Military and Paramilitary Activities in and against Nicaragua (Nicaragua v United States of America) (Merits) [1986] ICJ Rep 14.

• North Sea Continental Shelf Cases (Federal Republic of Germany v Denmark; Federal Republic of Germany v Netherlands) (Merits) [1969] ICJ Rep 3.

• Prosecutor v Hadžihasanović (Decision on Interlocutory Appeal Challenging Jurisdiction in Relation to Command Responsibility) ICTY-01-47 (16 July 2003).

• Question of the Delimitation of the Continental Shelf between Nicaragua and Colombia beyond 200 Nautical Miles from the Nicaraguan Coast (Nicaragua v Colombia) (Judgment) (13 July 2023) General List No 154.

• Right of Passage over Indian Territory (Portugal v India) (Merits) [1960] ICJ Rep 6.

• Sedco Incorporated v National Iranian Oil Company and Iran (Second Interlocutory Award) (27 March 1986) (1987) 10 Iran-US CTR 180.

Further Cases

• Dispute regarding Navigational and Related Rights (Costa Rica v Nicaragua) (Merits) [2009] ICJ Rep 213.

• Prosecutor v Tadić (Decision on the Defence Motion for Interlocutory Appeal on Jurisdiction) IT-94-1-AR72 (2 October 1995).

• Territorial Dispute (Libyan Arab Jamahiriya/Chad) (Merits) [1994] ICJ Rep 6.