Metadata is literally ‘data about data’. In the context of meteorological records, metadata covers the description of the observation site and its surroundings, the instruments in use and their units together with any significant changes over time, and any other relevant information, such as where the site’s records are archived. Its importance lies in providing essential information for any other user of the records to understand more about the location and characteristics of the data, and therefore enables more informed use of the data. Metadata are especially important for elements which are particularly sensitive to exposure, such as precipitation, wind and temperature, and for professional sites, especially so for long-period records. This chapter sets out what should be recorded, following guidelines laid down by the World Meteorological Organization in the so-called CIMO guide (Commission for Instruments and Methods of Observation).