Introduction. The article dwells on the international legal cooperation on resolution of the consequences of dual and multiple nationality concerning extradition of bi- and polypatrids, resolution of investment disputes with their participation and provision of diplomatic protection. Theoretical Basis. Methods. The theoretical basis of the present study was based on doctrinal and legal sources, as well as legal precedents in the field of extradition of persons with dual and multiple nationality, resolution of investment disputes, and diplomatic protection. Formal-legal, comparative-legal, historical, and predictive methods were used in the research. Results. Dual and multiple nationality appear more and more widespread phenomena which is caused by the tendencies of modern interstate communication. In the absence of effective international legal regulation, in particular treaty regulation, a large number of disputes arises in connection with the legal status of bi- and polypatrids, among which the most urgent are those in the field of extradition, resolution of investment disputes and provision of diplomatic protection. The well-established approach in the area of bi- and polipatrizm implies the application of the principle of “effective nationality” in cases involving such persons. Discussion and Conclusion. The admissibility and conditions for application of effective nationality principle are frequently not stipulated in the international treaties (on extradition, on protection of investments, on dual nationality), therefore it seems advisable to supplement this kind of treaties with appropriate provisions on the procedure of effective nationality principle application.