This presentation explores the use of Uniform Resource Names (URNs), specifically URN:NBNs, as persistent identifiers (PID) in the repository context. URNs are a standards-based identifier system, which has been around since the 1990s. The National Library of Finland (NLF) has been providing a national URN resolution service at urn.fi since 2007. The ease of implementation and cost-free nature of URNs has led to the adoption of URN-based persistent addresses at all Finnish institutional repositories.URN:NBN identifiers are currently in active use in 13 European countries, with the national library usually providing services and coordination. However, as there is no organization overseeing the use of URNs on a global level, there has been little international cooperation in this area, and the profile of URN:NBN:s remains relatively low compared to some of the other PID systems, especially DOIs.As part of the EU-funded FAIRCORE4EOSC project, NLF has started to contact and interview the other national URN service providers. There are plans to foster collaboration by organising a URN:NBN webinar and producing a URN:NBN landscape study in 2024. The Meta Resolver service, which is being developed within the FAIRCORE4EOSC project, may also provide steps towards improved interoperability in PID resolution, including, importantly, URN:NBNs.