The ISSN International Centre is the registration authority for ISO 3297, which defines the International Standard Serial Number (ISSN). This persistent identifier is assigned to continuing resources, i.e. print and digital journals, websites, blogs and repositories. The metadata associated with this identifier is created by the ISSN network, which currently comprises 93 national centers and will celebrate its 50th anniversary in 2025. The data is published on a portal based on a freemium model (https://portal.issn.org) with core data made available for free. In its current strategy, the ISSN International Center has defined the interoperability of the ISSN with other persistent identifiers as a priority objective.