This book by Panos Merkouris is the public output of the ‘Rules of Interpretation of Customary International Law’ (TRICI-Law) research project. Its point of departure is a long-standing gap in international law scholarship: rules of customary international law are identified through State practice and opinio juris, yet their interpretation has attracted little academic attention. The author, a member of the European Research Council-funded TRICI-Law team, argues that the prevailing view—according to which the two-element identification test suffices and there is no distinct interpretive process—misconceives how customary international law functions. He contends that courts and States routinely interpret customary international law, often implicitly, and that a systematic framework for such interpretation is essential to enhance coherence, predictability and legitimacy in international law.

The book comprises ten chapters plus an annex. An introductory chapter sets out the puzzle and situates the debate within the International Law Commission’s (ILC) recent project on the identification of customary international law. Chapter 2 examines whether interpretation is precluded by the unwritten nature of custom. Through a close reading of ILC reports, State comments and secondary literature, Merkouris shows that the ILC conflated questions of existence and content and left unanswered whether and how rules may be interpreted once identified. He also refutes arguments that interpretation is impossible for unwritten norms, pointing out that unwritten international agreements and unilateral declarations have long been interpreted and that treating an identically worded treaty rule and customary international law rule differently would be illogical. Chapters 3 and 4 form the empirical core of the study. They survey the case law of international courts (including the International Court of Justice, the European Court of Human Rights, the International Criminal Court, the World Trade Organization and investment arbitration tribunals) and select domestic courts. By analysing dozens of decisions, the author demonstrates that judges do interpret customary norms, often by analogy with treaty interpretation, and sometimes without appreciating the distinction between identification and interpretation.

Chapter 5 addresses the ‘conundrum of in/correct interpretations’ (42). Merkouris argues that interpretation is not without limits but, rather, is constrained by the goals of coherence, systemic unity and the rule of law; he rejects a relativist view in which any plausible reading of State practice suffices. Chapters 6–8 explore limits on interpretation. Chapter 6 proposes general precautions, warning that too great an interpretive freedom might blur the line between interpretation and law-making. Chapter 7 examines jus cogens as an external limit: customary rules must not be interpreted to infringe peremptory norms. Chapter 8 discusses ‘revision’—when interpretation effectively changes a rule’s content, and stresses that interpretation must not become de facto law-making. Chapter 9 then considers misinterpretation, distinguishing legitimate development from distortion; examples from international and domestic judgments illustrate how courts sometimes misapply interpretive principles. The concluding chapter summarises the findings and calls for greater attention to interpretive methodology in both scholarship and practice. An annex reproduces the TRICI-Law Draft Guiding Principles on the Interpretation of Customary International Law (Draft Principles).

Merkouris’s central contribution is to disentangle identification from interpretation of customary international law and to show that courts already interpret customary rules even though they rarely articulate a methodological framework for doing so. By marshalling a broad body of jurisprudence, he demonstrates that interpretation is not only possible but often necessary when rules are general or vague. The book also highlights the dangers of ignoring interpretive questions: courts risk conflating the development of the law with its identification, and the absence of a shared methodology undermines coherence and predictability. The annexed Draft Principles aim to fill this gap by adapting tools from treaty interpretation (text, context, object and purpose, systemic coherence, subsequent practice) to the customary realm.

The argument that interpretation is indispensable for customary international law is convincingly made. The author persuasively criticises the view that unwritten rules cannot be interpreted and shows that courts routinely perform interpretive exercises. The engagement with ILC materials and State comments is thorough and up to date, and the case law survey is impressively broad. The normative stance—that a recognised methodology would enhance legitimacy and coherence—is compelling, and the annexed Draft Principles provide a useful starting point.

Nevertheless, certain limitations warrant a mention. First, the book’s brevity inevitably constrains its analytical depth. The chapters summarise large bodies of case law, but many decisions are discussed only briefly; readers seeking detailed doctrinal analysis may need to consult the cases themselves. Second, the work adopts a primarily doctrinal and empirical approach. While this is understandable given its aims, it leaves normative questions underexplored. For instance, the criteria for distinguishing interpretation from revision are outlined but not fully theorised; whether courts should have different interpretive techniques for customary international law compared with treaties could have been further developed. Third, the book focuses mainly on Western jurisprudence. Although some domestic cases from Asia, Africa and Latin America are included, a more systematic examination of non-Western perspectives might have enriched the analysis.

The book is clearly written and well organised. The chapters follow a logical progression, and signposting allows readers to follow the argument easily. The author strikes a balance between descriptive exposition and normative argument. References to cases and literature are plentiful and current, demonstrating the author’s command of the field. Occasionally the prose is dense—especially when summarising multiple cases in quick succession—but this is mitigated by helpful headings and a comprehensive table of contents.

Interpretation of Customary International Law: Of Methods and Limits makes a timely and valuable contribution to the study of sources doctrine. By exposing the interpretive practices already operating in international and domestic courts, Merkouris undermines the notion that the identification of customary international law suffices and invites scholars and practitioners to develop a coherent interpretive methodology for its interpretation. The book’s succinctness and open-access format make it a convenient entry point for readers interested in customary international law and its future development. While some areas could have been explored further, the volume succeeds in filling a significant gap in the literature and will serve as a springboard for future research on the interpretation of customary norms.