State responsibility in international law is considered one of the cornerstones of the field. For a long time it remained the exclusive responsibility system due to the primacy of States as subjects of international law. Its unique position has nonetheless been challenged by several developments both within and outside the international legal order, such as the rise of alternative responsibility ideas and practices, as well as globalization and its consequences. This book adopts a critical and holistic approach to the law of State responsibility and analyzes the functionality of the general rules of State responsibility in a changed international landscape characterized by the fragmentation of responsibility. It is argued that State responsibility is not equally relevant across the broad spectrum of international obligations, and that alternative constructions of responsibility, namely international criminal law and international liability, have increased in standing.