This book offers a comprehensive analysis of the international law applicable to cyber operations, including a systematic examination of attribution, lawfulness and remedies. It demonstrates the importance of countermeasures as a form of remedies and also shows the limits of international law, highlighting its limits in resolving issues related to cyber operations. There are several situations in which international law leaves the victim State of cyber operations helpless. Two main streams of limits are identified. First, in the case of cyber operations conducted by non-state actors on the behalf of a State, new technologies offer various ways to coordinate cyber operations without a high level of organization. Second, the law of State responsibility offers a range of solutions to respond to cyber operations and seek reparation, but it does not provide an answer in every case and it cannot solve the problem related to technical capabilities of the victim.