This article describes some of the challenges raised by multinational operations for the application of international humanitarian law. Such challenges are the result of different levels of ratification of treaties, divergent interpretations of shared obligations, and the fact that there is no central authority that determines who is a party to an armed conflict. The article discusses methods that have been developed to ensure 'legal interoperability'. Some of these methods attempt to avoid situations where such interoperability is required. Where this is not possible, a 'maximalist' or a 'minimalist' approach can be taken, and in practice these are usually combined. Keywords: international humanitarian law, multinational forces, multinational operations, interoperability.