What does responsibility mean in International Relations (IR)? This handbook brings together cutting-edge research on the critical debates about responsibility that are currently being undertaken in IR theory. This handbook both reflects upon an emerging field based on an engagement in the most crucial theoretical debates and serves as a foundational text by showing how deeply a discussion of responsibility is embedded in broader questions of IR theory and practice. Contributions cover the way in which responsibility is theorized across different approaches in IR and relevant neighboring disciplines and demonstrate how responsibility matters in different policy fields of global governance. Chapters with an empirical focus zoom in on particular actor constellations of (emerging) states, international organizations, political movements, or corporations, or address how responsibility matters in structuring the politics of global commons, such as oceans, resources, or the Internet. Providing a comprehensive overview of IR scholarship on responsibility, this accessible and interdisciplinary text will be a valuable resource for scholars and students in many fields including IR, international law, political theory, global ethics, science and technology, area studies, development studies, business ethics, and environmental and security governance.