Panos Merkouris is Professor of International Law at the University of Groningen. He holds a Chair on Interpretation & Dispute Settlement in International Law. He is the Principal Investigator of the TRICI-Law project (ERC Grant Agreement No. 759728). Prof. Merkouris has written extensively on the law of treaties, sources and interpretation, most recently co-authoring Treaties in Motion (2020) with Prof. Malgosia Fitzmaurice. Andreas Kulick is Senior Research Fellow at the University of Tübingen, Germany and Visiting Professor at Albert Ludwigs University Freiburg in the summer semester of 2023. He has been a visiting fellow at the Lauterpacht Centre for International Law, NYU School of Law, and the European University Institute. He has written extensively on all aspects of international law, with a particular focus on international adjudication, and regularly advises and represents states in proceedings before international courts and tribunals, including in investment arbitration. José Manuel Álvarez-Zarate is Professor at Externado University of Colombia and Director of the Law and Economics Department where he runs the International Economic Law (IEL) program. He has written extensively on trade and investment matters and practices advising and representing in proceedings before local and communitarian tribunals, including in investment arbitration. Maciej Żenkiewicz is Assistant Professor at Nicolaus Copernicus University. He also teaches as a visiting professor at various universities including Antonio de Nebrija University, Spain, Pontificia Universidad Católica del Perú, Peru, and Xi'an Jiaotong University School of Law, China. He is the author of various articles and monographs. Konrad Turnbull is a PhD researcher within the Department of Transboundary Legal Studies and Faculty of Economics & Business at the University of Groningen. His core research focuses on international courts' approaches in adjudicating cases of structural discrimination & the public's perception of the efficacy of these institutions.

'The current volume is a thought-provoking publication that challenges preconceived ideas in relation to custom and international investment law. It analyses the continued relevance of custom and its interpretation in investment law. This publication not only broadens knowledge of investment law but also constitutes a very significant contribution to further understanding of intricacies and mysteries of customary international law. Authors of chapters are leading scholars and practitioners in field of international and investment law. They have dealt with complex and challenging aspects of uneasy relationship between international investment law and customary international law in an outstanding way. This volume is very highly recommended for everyone who is interested in challenging legal problems and evolution of international law.' Malgosia Fitzmaurice, Queen Mary University of London

• Foreword; Custom and international investment law Horacio A. Grigera Naón; Part I. Identifying Custom in International Investment Law: 1. The 'minimum standard of treatment' in international investment law: the fascinating story of the emergence, the decline and the recent resurrection of a concept Patrick Dumberry; 2. The recourse to legal experts for the establishment and interpretation of customary norms in investment law Saïda El Boudouhi; 3. The identification of customary international law and international investment law and arbitration: state practice in connection with investor-state proceedings Diego Mejía-Lemos; 4. Assessing damages in customary international law: the chorzów's tale José Manuel Álvarez-Zarate; Part II. The Interpretation of Secondary Rules in International Investment Law: 5. The uses of the work of the international law commission on state responsibility in international investment arbitration Sotirios-Ioannis Lekkas; 6. Revisiting the availability of countermeasures in investment arbitration Anna Ventouratou; 7. investment tribunals, the duty of compensation in cases of necessity: a customary law void? Federica I Paddeu; 8. 'A riddle wrapped in a mystery inside an enigma': equitable considerations in the assessment of damages by investment tribunals Emmanuel Giakoumakis; 9. Conflict of treaty norms and subsequent agreements in relation to the interpretation of treaties in international investment law Łukasz Kułaga; Part III. Interpreting Customary International Law: Current Challenges: 10. Police powers in a pandemic: investment treaty arbitration and the customary presumption of reasonable regulation Oliver Hailes; 11. Bilateral investment treaties, investor obligations and customary international environmental law Madhav Mallya; 12. The role of custom in international investment law remedies: the curious case of natural resources Filip Balcerzak; 13. A twail engagement with customary international investment law: some strategies for interpretation Nina Mileva; 14. Custom and its interpretation in international investment law: final musings Panos Merkouris, Andreas Kulick, José Manuel Álvarez-Zarate and Maciej Żenkiewicz.

Panos Merkouris is Professor of International Law at the University of Groningen. He holds a Chair on Interpretation & Dispute Settlement in International Law. He is the Principal Investigator of the TRICI-Law project (ERC Grant Agreement No. 759728). Prof. Merkouris has written extensively on the law of treaties, sources and interpretation, most recently co-authoring Treaties in Motion (2020) with Prof. Malgosia Fitzmaurice. Andreas Kulick is Senior Research Fellow at the University of Tübingen, Germany and Visiting Professor at Albert Ludwigs University Freiburg in the summer semester of 2023. He has been a visiting fellow at the Lauterpacht Centre for International Law, NYU School of Law, and the European University Institute. He has written extensively on all aspects of international law, with a particular focus on international adjudication, and regularly advises and represents states in proceedings before international courts and tribunals, including in investment arbitration. José Manuel Álvarez-Zarate is Professor at Externado University of Colombia and Director of the Law and Economics Department where he runs the International Economic Law (IEL) program. He has written extensively on trade and investment matters and practices advising and representing in proceedings before local and communitarian tribunals, including in investment arbitration. Maciej Żenkiewicz is Assistant Professor at Nicolaus Copernicus University. He also teaches as a visiting professor at various universities including Antonio de Nebrija University, Spain, Pontificia Universidad Católica del Perú, Peru, and Xi'an Jiaotong University School of Law, China. He is the author of various articles and monographs. Konrad Turnbull is a PhD researcher within the Department of Transboundary Legal Studies and Faculty of Economics & Business at the University of Groningen. His core research focuses on international courts' approaches in adjudicating cases of structural discrimination & the public's perception of the efficacy of these institutions.

'The current volume is a thought-provoking publication that challenges preconceived ideas in relation to custom and international investment law. It analyses the continued relevance of custom and its interpretation in investment law. This publication not only broadens knowledge of investment law but also constitutes a very significant contribution to further understanding of intricacies and mysteries of customary international law. Authors of chapters are leading scholars and practitioners in field of international and investment law. They have dealt with complex and challenging aspects of uneasy relationship between international investment law and customary international law in an outstanding way. This volume is very highly recommended for everyone who is interested in challenging legal problems and evolution of international law.' Malgosia Fitzmaurice, Queen Mary University of London

• Foreword; Custom and international investment law Horacio A. Grigera Naón; Part I. Identifying Custom in International Investment Law: 1. The 'minimum standard of treatment' in international investment law: the fascinating story of the emergence, the decline and the recent resurrection of a concept Patrick Dumberry; 2. The recourse to legal experts for the establishment and interpretation of customary norms in investment law Saïda El Boudouhi; 3. The identification of customary international law and international investment law and arbitration: state practice in connection with investor-state proceedings Diego Mejía-Lemos; 4. Assessing damages in customary international law: the chorzów's tale José Manuel Álvarez-Zarate; Part II. The Interpretation of Secondary Rules in International Investment Law: 5. The uses of the work of the international law commission on state responsibility in international investment arbitration Sotirios-Ioannis Lekkas; 6. Revisiting the availability of countermeasures in investment arbitration Anna Ventouratou; 7. investment tribunals, the duty of compensation in cases of necessity: a customary law void? Federica I Paddeu; 8. 'A riddle wrapped in a mystery inside an enigma': equitable considerations in the assessment of damages by investment tribunals Emmanuel Giakoumakis; 9. Conflict of treaty norms and subsequent agreements in relation to the interpretation of treaties in international investment law Łukasz Kułaga; Part III. Interpreting Customary International Law: Current Challenges: 10. Police powers in a pandemic: investment treaty arbitration and the customary presumption of reasonable regulation Oliver Hailes; 11. Bilateral investment treaties, investor obligations and customary international environmental law Madhav Mallya; 12. The role of custom in international investment law remedies: the curious case of natural resources Filip Balcerzak; 13. A twail engagement with customary international investment law: some strategies for interpretation Nina Mileva; 14. Custom and its interpretation in international investment law: final musings Panos Merkouris, Andreas Kulick, José Manuel Álvarez-Zarate and Maciej Żenkiewicz.