The article discusses the issues of substitution and equivalence of concepts in private international law, which were the subject of deliberations at the 73rd Session of the Institute of International Law in Santiago de Chile in 2007. It presents the Institute's position formulated in the resolution of October 27, 2007. Difficulties related to the application of foreign law are highlighted, as well as the need to seek functional similarities between domestic and foreign legal institutions to ensure the continuity and harmony of legal relationships. It is emphasized that substitution and equivalence do not require identical regulations, but only similarity of aims and interests.