The paper highlights of the current legal issues of applying countermeasures as a type of international responsibility. The study focuses on the fact that the concept of countermeasures and the mechanism of their application in modern international law is undergoing transformation and modernisation. Attention is drawn to the urgency of solving the legal problems identified by this study, which is inextricably linked to the conduct of hostilities in Ukraine. Among the legal issues which need to be urgently addressed by legal science and practice, the paper lists the following: formation of a modern concept of countermeasures in international law, taking into account the purpose of their application to the offending State, which is relevant to the time; construction of the so-called ‘roadmap’ for the application of countermeasures, in particular, with regard to the types and procedures which may be applied; use of countermeasures in cyberspace and use of collective countermeasures. The study substantiates the generalisation of the purpose of applying countermeasures as a way to encourage the offending State to fulfil its obligations to the international community. The paper proves the need to build such a ‘roadmap’ for the application of countermeasures, including the use of countermeasures in cyberspace. Attention is drawn to the recognition by legal science and practice of the possibility of applying collective countermeasures, subject to strict limitations in the application procedure. It is emphasised that the list of legal issues in the field of application of countermeasures in modern international law is not exhaustive. In modern international law, there are a number of legal and practical problems related to the application of countermeasures which need to be addressed through further scientific discussions and subsequent improvement of international norms, taking into account the problems outlined in this study.