Dr. Daniel G. Costelloe


Costelloe, Daniel

Daniel Costelloe is counsel in the Litigation/Controversy Department, and is a member of the International Arbitration Practice Group. Dr. Costelloe focuses his practice on international arbitration matters across a range of industries, including oil and gas, financial services and automotive. He has represented clients in arbitrations under a variety of arbitral rules and has particular experience with the ICC, ICSID and LCIA Rules. He also advises clients on public international law issues and in arbitration-related court proceedings.

Dr. Costelloe has also acted in pro bono matters with the Media Legal Defence Initiative, notably involving a challenge before the East African Court of Justice to a national law's compatibility with a state's treaty obligations. He has also advised a pro bono client on the international lawfulness of courts established by non-state actors.

Dr. Costelloe publishes on various topics within the field of public international law and has taught classes in public international law and guest classes on the law of treaties.

Professional Activities

  • American Society of International Law
  • Chatham House, Royal Institute of International Affairs

Honors & Awards

Publications & News


September 7, 2017

Legal Consequences of Peremptory Norms in International Law

The book, authored by Daniel Costelloe and published by Cambridge University Press, explores peremptory norms' legal effects across general international law, including their effects for treaties, UN Security Council resolutions, certain unilateral acts on the part of states, state responsibility and the responsibility of international organizations, and state immunity.

December 5, 2016

Daniel Costelloe Earns Prestigious Young Scholar Award

Daniel Costelloe has been awarded the International and Comparative Law Quarterly 2016 Young Scholar Prize for his article "Treaty Succession in Annexed Territory."

August 11, 2016

Interpretation of Secondary Instruments in International Law

The article, co-authored by Daniel Costelloe with Professor Malgosia Fitzmaurice of Queen Mary University and published in the Polish Yearbook of International Law, explores the legal principles that govern the interpretation of instruments adopted by a body empowered by a treaty to take action with respect to the treaty, but which are not themselves a treaty (such as U.N. Security Council resolutions or International Seabed Authority regulations).

May 16, 2016

Lawmaking by treaty: Conclusion of treaties and evolution of treaty regimes in practice

A chapter co-authored by Daniel Costelloe, published in Research Handbook on the Theory and Practice of International Lawmaking (Edward Elgar, 2016). The chapter explores the formal steps under the Vienna Convention on the Law of Treaties leading to the conclusion of a treaty and the manner in which a treaty regime can subsequently develop in practice.

April 4, 2016

Treaty Succession in Annexed Territory

The article, authored by Daniel Costelloe and published by International & Comparative Law Quarterly, explores the question whether the territorial application of a treaty extends to unlawfully annexed territory. It examines possible bases for the application of the annexing state's treaties, as well as the status of the dispossessed state's treaties, in annexed territory, both under the law of treaties and the law of state succession.

May 28, 2015

WilmerHale Achieves Victory in Development for Free Speech Rights in East African Community Partner States

On May 15, 2015, WilmerHale achieved a significant victory on behalf of the Burundian Journalists' Union in a case before the East African Court of Justice. The decision represents an important development for free speech rights in East African Community Partner States.

Speaking Engagements


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PhD, Law, University of Cambridge, 2014, WM Tapp Studentship in Law, Managing Editor and Blog Editor, Cambridge Journal of International and Comparative Law

LLM, Harvard Law School, 2010, Line Editor, Harvard International Law Journal

LLB (Law with French Law), first class, University College London, 2009, Andrews Medal, Hurst Prize, Richardson Prize

Bar Admissions

New York




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