OverviewMetadata models help to give an understanding of the development of metadata standards. The chapter starts with an overview of data modelling and its relationship to metadata. It defines some of the terminology used to describe modelling languages, using the Unified Modelling Language (UML) as an example. Systems such as RDF and the ABC Ontology are discussed before considering domain-specific modelling frameworks such as the Library Reference Model (LRM), indecs for the book trade and OAIS for online exchange of information. Van Hooland and Verborgh (2014) talk about four types of modelling: tabular, relational, meta mark-up and RDF. This chapter focuses on the last two types.