The ISSN system is a well-known and well-established system for the identification of serial publications.The ISSN Network is co-ordinated by the International Centre (IC) based in Paris, and there are over 70 ISSN National Centres worldwide. In the world of electronic resources there is an increased value of the ISSN as a persistent identifier, and the assignment of ISSNs to online resources is strongly encouraged by the IC. However, anecdotal evidence had suggested that this might not be consistent throughout the ISSN Network. The aim of the research project was to explore any differences and variations in practice that may exist between the National Centres, and the reasons for these. The research was undertaken using four National Centres as case studies and three main areas of research were undertaken to produce data. These were observation, interviewing and documentary analysis. The research was carried out using a theoretical framework generated by the ideas of Bowker and Star (1999).