This book provides an in-depth analysis of equality of parties in international adjudication beyond the conventional interstate framework. It challenges the idea that party equality in international proceedings simply follows from sovereign equality and demonstrates that this assumption is ill-suited to disputes involving private actors. It argues that equality can be protected in asymmetric proceedings between states or international organizations and private entities, even when the parties are not formally equal in the international legal order. Drawing on investor-state arbitration, regional human rights courts, and international administrative tribunals, the book identifies a common legal foundation for equality of parties in a general principle of international law derived from national approaches to administrative adjudication.