León Castellanos-Jankiewicz is senior researcher in international law at the University of Amsterdam Faculty of Law and the Asser Institute for International and European Law in The Hague. He regularly advises governments on matters of international law, and has extensive experience working in projects involving interdisciplinary research, multistakeholder engagement and advocacy. 

His research covers three broad fields. First, he is interested in arms trade and weapons transfer policy, in particular human rights safeguards, export controls, corporate accountability and due diligence. Second, he looks at the governance of global public goods through private law, including strategic litigation and supply chain resilience. Finally, he conducts research on legal history, focusing on the intersection of ideas between public and private international law.

Castellanos-Jankiewicz is principal investigator of the RELY Project: Rearming Europe with Legal Accountability, which develops actionable legal safeguards and human rights risk mitigation strategies at the European Union level to prevent corporate and state misconduct for wrongful weapons transfers as the bloc ramps up its defense capabilities.

He has been awarded competitive research grants by the Swiss National Science Foundation, the U.S. State Department and the Dutch Research Council (NWO), and has extensive teaching experience lecturing in English, Spanish and French. He frequently writes for print and online publications, including El País, NRC and Just Security and is a listed expert in the Forum on the Arms Trade. He sits on the Advisory Committee of Global Action on Gun Violence and is a regular commentator in the media.

Previously, he was Max Weber Postdoctoral Fellow at the European University Institute (2017-18) and has held visiting positions at the University of Giessen (2023; 2025), the University of Cambridge (2017) and Harvard Law School (2015-16). He is the inaugural recipient of the David D. Caron Prize of the American Society of International Law (2019) and holds a PhD in International Law from the Geneva Graduate Institute. 

• 'The responsibility of gunmakers doesn't end at the factory gate', El País English, June 11, 2025.

• 'Google worried it couldn't control how Israel uses project Nimbus, files reveal', The Intercept, May 13, 2025.

• 'Can Mexico win its battle with US gun companies?', BBC World Service, ‘The Inquiry’, March 7, 2024.

• 'Mexico wins support for suit against gunmakers', China Daily, April 24, 2023.