This chapter gives an overview of the most important metadata formats and the way they treat subject indexing. Libraries used to be the primary source of structured metadata, but in the last decades of the twentieth century new standards successfully emerged in different environments and also made their way into the library world. Some metadata formats provide facilities to store detailed and structured subject information; others leave a lot of freedom to the user. The multitude of formats make conversion from one into another necessary, which may result in loss of structure. Specific instruments were developed to transfer data from one format into another: frameworks, registers and crosswalks.