What does accountability of international organisations mean? What does it mean for transnational corporations? How can they be held accountable? This book analyses and compares the accountability frameworks of both types of institutions—their rationales, conceptualisations and mechanisms. To achieve this, it comprehensively examines two select institutions: the United Nations and Siemens. It presents their similarities and differences in detail, compares their accountability mechanisms and critically assesses their conceptualisations of accountability. To understand underlying structures, the book makes use of economic theories. It adopts and refines a procedural understanding of accountability originally developed in international relations and political science. Last but not least, this book examines to what extent accountability has become a legal concept and aims at contributing to the ongoing efforts of conceptualising ‘accountability’.