Identifiers are character strings that uniquely identify (digital) objects. Objects can be datasets (DOI or RADAR-ID), persons (ORCID) or terms from a controlled vocabulary (Funder-ID). Some identifiers (so-called persistent identifiers) refer permanently to the assigned object and remain stable over time. Identifiers enable the reliable and permanent addressing and disambiguation of objects. They support interoperability and thus facilitate findability and citation of research data.