have generally declined in consequence. However, many problems remain because originators often fail to produce online reports or publications to an adequate bibliographic standard (often omitting a publication date, for instance). Documents are often not assigned permanent URLs or DOI numbers, or stored in electronic depositories, so that link rot can develop within citations, reference lists, databases and websites. Copyright law and the copyrighted status of many reports inhibits their downloading and electronic storage and there is a lack of large scale collecting of digital grey literature. Securing long-term access to and management of grey literature in the digital era thus remains a considerable problem. The amount of digital grey literature now available also poses a problem for finding relevant resources and to be able to assess their credibility and quality given the number of resources now available. At the same time a great deal of grey literature remains hidden, either not made public or not made discoverable via search engines. Databases Various databases and libraries collect and make available print and digital grey literature; however, the cost and difficulty of finding and cataloguing grey literature mean that it is still difficult to find large collections. The British Library began collecting print grey literature in the post-WWII period and now has an extensive collection of print resources. Analysis & Policy Observatory has an extensive collection of grey literature on a wide range of public policy issues, ArXiv is a collection of preprints on physics and other sciences, and RePEc is a collection of economics working papers. Many university libraries provide subject guides that give information on grey literature and suggestions for databases. ROAR and OpenDOAR are directories of open access institutional repositories and subject repositories, many of which contain some grey literature. Several academic search engines exist to combine the open data provided by such open archives through OAI-PMH, as well as records from publishers deposited in CrossRef and other sources. They include BASE, CORE and Unpaywall, which indexes over 20 million open access publications as of 2020. Resources and advocacy The annual international grey literature conference series has been organized since 1993 by the Europe-based organization GreyNet. Research in this field of information has been systematically documented and archived via the International Conference on Grey Literature series. Greynet also produces a journal on grey literature and has been a key advocate for the recognition and study of grey literature,