¹ Statute of the International Court of Justice (entered into force 24 October 1945) 1 UNTS XVI, art 38(1).

² Lepard, Brian D, ‘Introduction’ in Lepard, Brian D (ed), Reexamining Customary International Law (Cambridge University Press 2017) 1, 2CrossRefGoogle Scholar. See also Venzke, Ingo, How Interpretation Makes International Law: On Semantic Change and Normative Twists (Oxford University Press 2012) 20CrossRefGoogle Scholar; The Case of the SS Lotus (France v Turkey) (1927) PCIJ Rep (Ser A, No 10) [41]–[45].

³ Clapham, Andrew, Brierly's Law of Nations (Oxford University Press 2012) 63–64CrossRefGoogle Scholar. The notion of ‘general principles of law’ has raised different interpretations. Although it is beyond the scope of this review to address this source in detail, it should be noted that the prevailing view is that these principles are, first, expressions of national legal systems and, second, expressions of other ‘unperfected sources of international law enumerated in … conventions, customs, writings of scholars,’ as well as decisions of the ICJ and other international tribunals. These expressions, along with manifestations of international consensus set out in United Nations (UN) General Assembly and Security Council Resolutions, may singularly or cumulatively with others be considered to be expressions of a given principle: Bassiouni, Cherif, ‘Functional Approach to “General Principles of International Law”’ (1989–90) 11 Michigan Journal of International Law 768, 768–69Google Scholar. See also Thomas Kleinlein, ‘Customary International Law and General Principles: Rethinking Their Relationship’ in Lepard (n 2) 131, 134. There, Kleinlein addresses two existing views on how to interpret the phrase ‘general principles of law recognised by civilised nations’, one narrow, the other broader. While the former holds that such principles must be recognised in municipal law, the broader alternative distinguishes three categories of principles on the basis of their origin: (i) principles stemming from domestic law; (ii) general principles originating in international relations; and (iii) general principles applicable to all kinds of legal relations.

⁴ Cassese, Antonio, International Law (Oxford University Press 2005) 154Google Scholar. These elements were confirmed recently in International Law Commission (ILC), Identification of Customary International Law (30 May 2016), UN Doc A/CN.4/L.872, Draft Conclusion 2 [3].

⁵ North Sea Continental Shelf Cases (FRG v Denmark; FRG v Netherlands), Judgment [1969] ICJ Rep 43 [74].

⁶ Michael Wood, ‘Foreword’ in Lepard (n 2) xiii, xv.

⁷ In 2013, the ILC decided to change the title of the topic from ‘Formation and Evidence of Customary International Law’ to ‘Identification of Customary International Law’: UN General Assembly, Report of the International Law Commission, UN Doc A/68/10 (2013), 5–6.

⁸ d'Aspremont, Jean and Besson, Samantha, The Oxford Handbook of the Sources of International Law (Oxford University Press 2017)Google Scholar.

⁹ Henckaerts, Jean-Marie and Doswald-Beck, Louise, Customary International Humanitarian Law, Vol I: Rules (Cambridge University Press/International Committee of the Red Cross 2005)CrossRefGoogle Scholar.

¹⁰ Lepard, Brian D, Customary International Law: A New Theory with Practical Applications (Cambridge University Press 2010)CrossRefGoogle Scholar.

¹¹ See, among many others, Bradley, Curtis A (ed), Custom's Future: International Law in a Changing World (Cambridge University Press 2016)CrossRefGoogle Scholar, in which Lepard himself is responsible for one of the chapters; Arajärvi, Noora, The Changing Nature of Customary International Law: Methods of Interpreting the Concept of Custom in International Criminal Tribunals (Routledge 2014)CrossRefGoogle Scholar; Bederman, David J, Custom as a Source of Law (Cambridge University Press 2010)CrossRefGoogle Scholar; Schlütter, Birgit, Developments in Customary International Law: Theory and the Practice of the International Court of Justice and the International Ad Hoc Criminal Tribunals for Rwanda and Yugoslavia (Martinus Nijhoff 2010)CrossRefGoogle Scholar; Sender, Omri and Wood, Michael, ‘The Emergence of Customary International Law: Between Theory and Practice’ in Brölmann, Catherine and Radi, Yannick (eds), Research Handbook on the Theory and Practice of International Lawmaking (Edward Elgar 2016)Google Scholar; Carty, Anthony, ‘Scandinavian Realism and Phenomenological Approaches to Statehood and General Custom in International Law’ (2003) 14 European Journal of International Law 817CrossRefGoogle Scholar; Vagts, Detlev F, ‘International Relations Looks at Customary International Law: A Traditionalist's Defence’ (2004) 15 European Journal of International Law 1031CrossRefGoogle Scholar; Baker, Roozbeh (Rudy) B, ‘Customary International Law in the 21st Century: Old Challenges and New Debates’ (2010) 21 European Journal of International Law 173CrossRefGoogle Scholar.

¹² Kleinlein (n 3) 158.

¹³ Jean-Marie Henckaerts and Els Debuf, ‘The ICRC and the Clarification of Customary International Humanitarian Law’ in Lepard (n 2) 161.

¹⁴ Noora Arajärvi, ‘From the “Demands of Humanity”: The Formulation of Opinio Juris in Decisions of International Criminal Tribunals and the Need for a Renewed Emphasis on State Practice’ in Lepard (n 2) 189.

¹⁵ Brian D Lepard, ‘Toward a New Theory of Customary International Human Rights Law’ in Lepard (n 2) 233.

¹⁶ Anna Williams Shavers, ‘Using Customary International Law to Improve Women's Lives’ in Lepard (n 2) 266, 268. In this sense, the author claims that ‘[s]ignificant evidence shows that even in the absence of widespread or universal practice, states believe that freedom from sex inequality is a desirable norm to implement now or in the near future, and is thus a customary norm’: ibid 306.

¹⁷ Sofia Michaelides-Mateou, ‘Customary International Law in Aviation: A Hundred Years of Travel through the Competing Norms of Sovereignty and Freedom of Overflight’ in Lepard (n 2) 309; and Frans G von der Dunk, ‘Customary International Law and Outer Space’ in Lepard (n 2) 346.

¹⁸ Lepard, ‘Introduction’ in Lepard (n 2) 1, 42; see also Clapham (n 3) 60.

¹⁹ Michaelides-Mateou (n 17) 344.

²⁰ von der Dunk (n 17) 372.

²¹ Brian D Lepard, ‘Concluding Reflections’ in Lepard (n 2) 377, 394.

²² Kleinlein (n 3) 133.

²³ Hollis, Duncan B, ‘Why State Consent Still Matters – Non-State Actors, Treaties, and the Changing Sources of International Law’ (2005) 23 Berkeley Journal of International Law 137, 141Google Scholar.

²⁴ d'Aspremont, Jean, Formalism and the Sources of International Law: A Theory of the Ascertainment of Legal Rules (Oxford University Press 2011) 149CrossRefGoogle Scholar.

²⁵ Jennings, Robert Y, ‘What is International Law and How Do We Tell It when We See It?’ in Koskenniemi, Martti, Sources of International Law (Routledge 2000) 27, 29Google Scholar.

²⁶ Lepard (n 18) 18. See also Kälin, Walter and Künzli, Jörg, The Law of International Human Rights Protection (Oxford University Press 2009) 67Google Scholar.

²⁷ Roberts, Anthea, ‘Traditional and Modern Approaches to Customary International Law: A Reconciliation’ (2001) 95 American Journal of International Law 757CrossRefGoogle Scholar.

²⁸ Lepard (n 18) 1–44. One can identify other challenges, such as how to determine the existence of opinio juris, the function of state practice, the definition of jus cogens customary norms, and the relationship between customary law and ethics. In the same sense, while D'Amato has lamented the ‘tremendous amount of disagreement among scholars and publicists over the rules of customary international law’ and the lack of a ‘consistent theory of custom’, Koskenniemi has explained that ‘modern legal argument lacks a determinate, coherent concept of custom. Anything can be argued so as to be included within it as well as so as to be excluded from it’: D'Amato, Anthony, The Concept of Custom in International Law (Cornell University Press 1971) 5Google Scholar; Koskenniemi, Martti, From Apology to Utopia (Cambridge University Press 2005) 409Google Scholar. See also Sender, Omri and Wood, Michael, ‘A Mystery No Longer? Opinio Juris and Other Theoretical Controversies Associated with Customary International Law’ (2017) 50 Israel Law Review 299CrossRefGoogle Scholar.

²⁹ Some of these questions have led to several disagreements as to the value of customary law within the international realm: Shaw, Malcolm, International Law (6th edn, Cambridge University Press 2008) 73CrossRefGoogle Scholar.

³⁰ Lepard (n 18) 39–40.

³¹ Lepard (n 21) 377.

³² Lepard (n 18) 19. See also Lepard (n 10) 23–25 for different views on this issue. In the recent draft conclusions adopted by the ILC, the following were included as ‘Forms of practice’: ‘1. Practice may take a wide range of forms. It includes both physical and verbal acts. It may, under certain circumstances, include inaction. 2. Forms of state practice include, but are not limited to: diplomatic acts and correspondence; conduct in connection with resolutions adopted by an international organization or at an intergovernmental conference; conduct in connection with treaties; executive conduct, including operational conduct “on the ground”; legislative and administrative acts; and decisions of national courts. 3. There is no predetermined hierarchy among the various forms of practice’: ILC (n 4) Draft Conclusion 6 [7].

³³ D'Aspremont (n 24) 163.

³⁴ Lepard (n 10) 1, 9.

³⁵ ibid 14.

³⁶ Lepard (n 18) 16–17.

³⁷ ibid.

³⁸ The Paquete Habana v United States 175 US 677 (1900), para 700.

³⁹ North Sea Continental Shelf Cases (n 5) [77].

⁴⁰ Lepard (n 10) 16.

⁴¹ J Patrick Kelly, ‘Customary International Law in Historical Context: The Exercise of Power without General Acceptance’ in Lepard (n 2) 47, 47.

⁴² ibid 85.

⁴³ Other authors have also addressed different motivations to follow by a given rule. When dealing with IHL, for instance, Krieger has noted that ‘[a]ctual decisions to obey a legal norm result from a complex mixture of diverse motivations. Power relations as well as historical, political, social and anthropological conditions determine these motivations so that compliance is context-dependent’: Krieger, Heike, ‘Introduction’ in Krieger, Heike (ed), Inducing Compliance with International Humanitarian Law: Lessons from the African Great Lakes Region (Cambridge University Press 2015) 4–5CrossRefGoogle Scholar. See also Jo, Hyeran, Compliant Rebels: Rebel Groups and International Law in World Politics (Cambridge University Press 2015) 77CrossRefGoogle Scholar.

⁴⁴ Fernando R Tesón, ‘Fake Custom’ in Lepard (n 2) 86.

⁴⁵ ibid 90. Interestingly, Tesón has criticised consent-based theories of international legal obligations based on the ground that consent could be immoral. As he explained, ‘the view that consent is the basis of international obligation is implausible [because] states may conclude immoral agreements and participate in immoral customs [Therefore] consent alone cannot be the basis of obligation’: Tesón, Fernando, A Philosophy of International Law (Westview Press 1998) 92Google Scholar.

⁴⁶ Niels Petersen, ‘The Role of Consent and Uncertainty in the Formation of Customary International Law’ in Lepard (n 2) 111.

⁴⁷ ibid 111–12.

⁴⁸ ibid 114.

⁴⁹ ibid 130.

⁵⁰ ibid.

⁵¹ Lepard (n 21) 383.

⁵² Crawford, James, Brownlie's Principles of Public International Law (Oxford University Press 2012) 28CrossRefGoogle Scholar.

⁵³ ibid.

⁵⁴ Klabbers, Jan, International Law (Cambridge University Press 2013) 30CrossRefGoogle Scholar.

⁵⁵ Kelly (n 41) 80. The inconsistencies of the persistent objector principle have also been addressed by other authors: eg, D'Aspremont (n 24) 165–66; and Dumberry, Patrick, ‘Incoherent and Ineffective: The Concept of Persistent Objector Revisited’ (2010) 59 International and Comparative Law Quarterly 779CrossRefGoogle Scholar.

⁵⁶ Kelly, ibid 82.

⁵⁷ Cassese, Antonio and Weiler, Joseph (eds), Change and Stability in International Law-Making, Series A, Vol 9 (European University Institute 1988)CrossRefGoogle Scholar.

⁵⁸ Antonio Cassese, ‘General Round-Up’ in Cassese and Weiler, ibid 23. Weiler affirms in this sense that ‘[i]t is the old family of nations, the First World … that is now insistent on the “persistent objector” and is “anti-universalist”. Of course, this is a pure reflection of politics. You insist on the “persistent objector” when you are in a minority, and you accept the universal principle when you feel that you control it. In the fifties in the United Nations the Occidental World dominated the scene and was in a position to insist on universalism. Now the tables have been turned’.

⁵⁹ Antonio Cassese, ‘The Classical “Sources” of International Law Revisited’ in Cassese and Weiler (n 57) 12.

⁶⁰ Mendelson, Maurice, ‘The Formation of Customary International Law’ (1998) 272 Collected Courses of the Hague Academy of International Law 228Google Scholar.

⁶¹ ibid 239.

⁶² Petersen (n 46) 120, 129. See, in a similar sense, Lepard (n 10) 333–36.

⁶³ Henckaerts and Debuf (n 13) 187. On jus cogens rules and customary international law, see also Kolb, Robert, Peremptory International Law Jus Cogens: A General Inventory (Hart 2015) 64–72Google Scholar.

⁶⁴ Lepard (n 10) 229.

⁶⁵ ibid.

⁶⁶ Klabbers (n 54) 31, fn 31.

⁶⁷ Kelly (n 41) 82.

⁶⁸ Boyle, Alan, ‘International Lawmaking: Towards a New Role for the Security Council?’ in Cassese, Antonio (ed), Realizing Utopia: The Future of International Law (Oxford University Press 2012) 172, 181–82Google Scholar.

⁶⁹ ILC (n 7) Draft Conclusion 15 [16] (1). See also Sender and Wood (n 28) 307–08, where the authors also reaffirm that the draft conclusion stating the persistent objector rule was widely supported and met with only limited opposition.

⁷⁰ For a recent publication on the topic, see Green, James A, The Persistent Objector Rule in International Law (Oxford University Press 2016)Google Scholar.

⁷¹ Tesón (n 44) 87.

⁷² ibid 88.

⁷³ Monica Hakimi, ‘Custom's Method and Process: Lessons from Humanitarian Law’ in Bradley (n 11) 148, 153.

⁷⁴ As Tesón correctly affirms, this is a variation of the view that if many states adopt a rule, it then becomes binding on those states that have rejected that same rule: Tesón (n 44) 92–93.

⁷⁵ ibid 97.

⁷⁶ Some may claim, however, that the question of the customary status of IHRL rules has become less significant as a result of the number of treaty ratifications. This debate is nevertheless relevant in a number of areas: in this respect see Clapham, Andrew, Human Rights Obligations of Non-State Actors (Oxford University Press 2006) 85–87CrossRefGoogle Scholar; and Kälin and Künzli (n 26) 67–68.

⁷⁷ Tesón (n 44) 106.

⁷⁸ ibid 107.

⁷⁹ Lepard (n 15) 239.

⁸⁰ Case concerning Armed Activities on the Territory of the Congo (DRC v Rwanda), Judgment [2006] ICJ Rep 6 [64], affirming that it ‘is assuredly the case’ that the prohibition of genocide is a norm of jus cogens. See also Reservations to the Convention on the Prevention and Punishment of the Crime of Genocide, Advisory Opinion [1951] ICJ Rep 23 [2], noting that ‘the principles underlying the [Genocide] Convention are principles which are recognized by civilized nations as binding on States even without any conventional obligation’. Interestingly, Meron affirmed almost thirty years ago that ‘[o]ver the years, the Court has inquired into the existence of state practice and of opinio juris with varying degrees of detail ranging from the specific … to the brief and conclusory’. The ICJ's more recent view ‘accords limited significance to state practice, especially to inconsistent or contrary practice, and attributes central normative significance to resolutions both of the United Nations General Assembly and of other international organizations … The burden of proof to be discharged in establishing custom in the field of human or humanitarian rights is thus less onerous than in other fields of international law’: Meron, Theodor, Human Rights and Humanitarian Norms as Customary Law (Oxford University Press 1991) 108CrossRefGoogle Scholar.

⁸¹ Tesón (n 44) 106. For some recent studies on how the ICJ identifies CIL, see Tomka, Peter, ‘Custom and the International Court of Justice’ (2013) 12 The Law and Practice of International Courts and Tribunals 195–216CrossRefGoogle Scholar; Petersen, Niels, ‘The International Court of Justice and the Judicial Politics of Identifying Customary International Law’ (2017) 28 European Journal of International Law 357CrossRefGoogle Scholar; Talmon, Stefan, ‘Determining Customary International Law: The ICJ's Methodology between Induction, Deduction and Assertion’ (2015) 26 European Journal of International Law 417CrossRefGoogle Scholar.

⁸² Tesón (n 44) 106.

⁸³ Sandesh Sivakumaran, ‘Who Makes International Law? The Case of the Law of Armed Conflict’, 7 December 2017, 6, https://papers.ssrn.com/sol3/papers.cfm?abstract_id=3084238.

⁸⁴ Lepard (n 15) 233.

⁸⁵ ibid 241.

⁸⁶ Luigi Condorelli, ‘Customary International Law: The Yesterday, Today, and Tomorrow of General International Law’ in Cassese (n 68) 147, 151.

⁸⁷ Petersen, Niels, ‘Customary Law Without Custom? Rules, Principles, and the Role of State Practice in International Norm Creation’ (2007) 23 American University International Law Review 275Google Scholar.

⁸⁸ Lepard (n 10) 8, 97–98; see also Lepard (n 15) 252.

⁸⁹ The role of ethical values is also addressed in Petersen (n 46) 127–30.

⁹⁰ Petersen, Niels, ‘Brian D. Lepard, Customary International Law: A New Theory with Practical Application’ (2010) 21 European Journal of International Law 795CrossRefGoogle Scholar.

⁹¹ This should not come as a surprise, considering that Lepard had already rejected the participation of non-state actors in his previous book. According to his definition, ‘opinio juris is an attitude among states regarding the desirability of instituting particular norms as legal norms’: Lepard (n 10) 186.

⁹² Lepard affirms that ‘for customary international law to retain its character as international law, we must focus on the views and practices of states – acknowledging, however, that these can be shaped by the programs and opinions of nongovernmental organizations and non-state actors. Indeed, we should be open to considering whether states may have changed their beliefs about desirable legal rules in light of these broader opinion trends within their countries and globally’: Lepard (n 21) 389–90.

⁹³ Many recent publications have dealt (or will deal) with the participation of non-state entities in the international realm: see Scobbie, Iain and Droubi, Sufyan (eds), Non-State Actors and the Formation of Customary International Law (Manchester University Press 2018 forthcoming)Google Scholar; d'Aspremont, Jean (ed), Participants in the International Legal System: Multiple Perspectives on Non-State Actors in International Law (Routledge 2011)CrossRefGoogle Scholar; Noortmann, Math, Reinisch, August and Ryngaert, Cedric (eds), Handbook on Non-State Actors in International Law (Hart 2015)Google Scholar; Noortmann, Math and Ryngaert, Cedric (eds), Non-State Actors Dynamics in International Law: From Law-Takers to Law-Makers (Ashgate 2010)Google Scholar.

⁹⁴ In his previous book, Lepard had actually recognised a role for other international bodies and tribunals. When addressing international courts, he explains that even though judicial determinations do not actually ‘create’ CIL or establish opinio juris – because the law would derive from the behaviour of states – the expertise of these organs can give their determination significant persuasive weight. Furthermore, decisions of international tribunals on the existence or non-existence of CIL will certainly affect the attitudes of states about the desirability ‘of instituting a particular legal norm – either confirming or rejecting the attitude of the tribunals’: Lepard (n 10) 184, and more broadly for other non-state actors 180–87, including the role of NGOs and UN institutions.

⁹⁵ Arajärvi (n 14) 198–99.

⁹⁶ Lepard (n 21) 392.

⁹⁷ Lepard (n 10) 180.

⁹⁸ Lepard (n 21) 392.

⁹⁹ UNGA Res 217A (III), UN Doc A/810 (1948) 71. For a detailed study of the customary law nature of the rights enshrined in the Universal Declaration of Human Rights, see Hannum, Hurst, ‘The Status of the Universal Declaration of Human Rights in National and International Law’ (1996) 25 Georgia Journal of International and Comparative Law 287–397Google Scholar. See also Clapham (n 76) 86.

¹⁰⁰ Henckaerts and Debuf (n 13) 179.

¹⁰¹ The study clarifies that the legal significance of such practice remains ‘unclear’, and it was only listed under the heading of ‘other practice’: Henckaerts and Doswald-Beck (n 9) xlii.

¹⁰² Henckaerts and Debuf (n 13) 181.

¹⁰³ ICTY, Prosecutor v Tadić, Decision on Defence Motion for Interlocutory Appeal on Jurisdiction, IT-94-1-I, Appeals Chamber, 2 October 1995.

¹⁰⁴ ibid [108].

¹⁰⁵ ibid [109].

¹⁰⁶ Arajärvi (n 11) 59.

¹⁰⁷ ILC (n 4) Draft Conclusion 4 [5].

¹⁰⁸ ILC, Second Report on Identification of Customary International Law by Michael Wood, Special Rapporteur (22 May 2014), UN Doc. A/CN.4/672, 30.

¹⁰⁹ ibid 33.

¹¹⁰ Sivakumaran (n 83) 21.

¹¹¹ ibid.

¹¹² Lepard (n 21) 393.

¹¹³ Condorelli (n 86) 150.

¹¹⁴ All of them, in fact, are also highlighted in the conclusion of the book: Lepard (n 21).

¹¹⁵ ibid 394.