Pierre's advisory and litigation experience cover land and maritime disputes, genocide, war reparation, human rights, aviation disputes, investment arbitration, State contracts, environmental claims, natural ressources, immunities of States and of international organizations.

Pierre is the author of over 100 published articles and book chapters, an award-winning book, a published course at The Hague Academy of International Law, a Massive Open Online Course (MOOC) on www.edx.org and courses on the United Nations Audiovisual Library of International Law. He is a member of the editorial boards of the Annuaire français de droit international, the Revue belge de droit international and the European Journal of International Law. He is currently the President of the European Society of International Law. 

A member of the Brussels Bar, Pierre argues international law cases before domestic courts and international tribunals. In particular, he has extensive experience before the International Court of Justice (ICJ), which he served previously as First Secretary. He appeared in more than ten cases before the ICJ and has also experience in interstate arbitration, investment arbitration and in human rights litigation, both before the European Court of Human Rights and the African Commission on Human and Peoples’ Rights. In addition to assisting and representing States, Pierre also represents international organizations before domestic courts and in arbitration proceedings. 

Pierre has been selected by the European Union as potential arbitrator under the Agreement on the withdrawal of the UK from the EU (reserve list). He is also retained by the EU as a potential arbitrator or a potential trade and sustainable development expert for dispute settlement panels under trade agreements to which the EU is a party.