Libraries have lost their former monopoly on creating metadata. Currently, the internet abounds in many non-library-related websites whose work is based on metadata resources that are sometimes created by merging (or aggregating) the resources of the largest libraries. In such a heterogeneous environment, it is of fundamental importance to find a solution to the problem of the interoperability of systems at all levels, including the metadata level. The interoperability of metadata - defined as the ability of two or more systems or their parts (using different hardware and software, with different data structures and interfaces) to exchange data with a minimal loss of content and functionality (NISO) - can be achieved in various ways, depending - among other things - on whether this process is planned before the creation of the metadata, or whether it concerns resources which have already been created in different systems and formats. There are two distinct approaches to metadata cooperation: one is traditional, authoritative and objectivist, while the other is constructivist and relates to the development of new information technologies. Sppecial attention has been paid not only to new possibilities, but also to the importance of the cooperation of metadata that results from the globalisation of the processes of its creation and the sharing of metadata in so-called data clouds (Linked Data).