The proliferation of private military companies (PMCs) represents one of the most significant phenomena in the transformation of contemporary warfare, raising fundamental questions about the applicability of existing international humanitarian law. This review article analyzes the legal framework for the engagement of private military companies in armed conflicts, with particular focus on evaluating the compliance of their activities with normative standards established by the Geneva Conventions of 1949 and their Additional Protocols of 1977. By applying the dogmatic-legal method, comparative analysis, and case studies, the research identifies key legal gaps in regulating the status of PMC personnel, determines problems in attributing responsibility for violations of international humanitarian law, and evaluates the effectiveness of existing oversight and sanctioning mechanisms. The innovative contribution of this research consists in the development of an original analytical model called the “Legal Responsibility Matrix for PMC Activities” (LRMA), which systematizes four dimensions of responsibility: state, corporate, individual, and command, and establishes criteria for their delineation and cumulative application in various operational scenarios. Research results show that the existing international legal framework, although formally applicable, suffers from structural deficiencies that enable de facto impunity for PMC personnel, and that it is necessary to draft a special international treaty that would explicitly regulate the status, rights, obligations, and responsibility of private military companies and their employees in the context of armed conflicts. The article concludes that the application of the proposed LRMA model is necessary to overcome the current normative fragmentation and establish a coherent system of legal responsibility that would guarantee effective protection of civilian population and prisoners of war in accordance with the fundamental principles of international humanitarian law.