Interoperability is one of the most important features of the modern information systems, concerning all aspects of their functioning, and particularly important in global networks. The article presents problems of metadata interoperability as an element of information systems’ interoperability, in particular digital libraries. Both types of interoperability are defined. The most important concepts of placing metadata interoperability among different types of information systems’ interoperability are discussed. The author lists the most popular methods of metadata interoperability on: schema, record, and repository levels. Finally, he develops a model of metadata interoperability referring to its’ semantics, syntactic, and pragmatics, and offers a set of tools used on the mentioned levels.