Armed conflict today involves a plethora of non-State actors including armed groups, private military companies, and hackers operating in the cyber domain. Frequently, these non-State actors act in a way that violates international law, for example by harming civilians, and they commonly act with the benefit of some form of State support. This raises critical questions about State responsibility. Attribution of Non-State Actor Conduct During Armed Conflict explores when and how international law attributes non-State actor conduct to a State, thereby triggering that State's international responsibility. Focusing on the six rules of attribution outlined in the International Law Commission's Articles on State Responsibility, the book provides a rigorous analysis of their practical application in the context of armed conflict. Using detailed case studies from the conflicts in Syria, Ukraine, and the Democratic Republic of the Congo, it examines each of the rules of attribution in turn, demonstrates when they would apply, and evaluates their effectiveness in holding States accountable for non-State actor conduct performed on their behalf. Critically assessing the legal and practical limitations of existing frameworks, the book argues that the predominant interpretation of the rules of attribution leaves an accountability gap, which enables States to act via proxy in a manner they could not lawfully operate via their own State organs. Combining doctrinal clarity with real-world relevance, it investigates how the inadequacies in the regulation of States' support to non-State actors in conflict situations should be remedied.