Metadata are a central component of today’s knowledge organization and are becoming increasingly important in the management of permanently growing amounts of information. This article elaborates on what is meant by the term metadata, which standards exist, and what metadata (standards) are used for. Furthermore, the focus is on indexing, which aims at a structured description of analog and digital resources. In order to ensure exchangeability between different institutions or data sources, common standards are required. In this context, it is important to have reference to controlled vocabularies and authority files, especially the Gemeinsame Normdatei (GND), which is key in the German-speaking world.