An identifier is a number or alphanumeric label that can be read either by machines or humans. It serves to make an object, a document, a person, a place or an organisation in both the real world and on the internet unique, permanently identifiable and retrievable. In order to achieve the aim of Open Science – the long-term and free availability of scientific information – it is important to ensure that the identifiers themselves are also open. They should therefore be based on an open, documented, free architecture, and be endorsed and promoted by the scientific community.

The most well-known and, for Open Science, most important Open Identifiers or “PIDs” (Permanent IDentifiers) are: DOIs, ORCID iDs, ISSNs (International Standard Serial Numbers) and the Handle System.