This entry provides an overview on the definition and evolution of grey literature in the emerging environment of digital resources and open access to scientific and technical information as well as the social sciences and humanities. First, it gives some empirical evidence on the importance of grey literature in scientific publications from different domains, especially library and information sciences (LIS), based on citation analysis. Other topics include the impact of Internet on the production of grey literature, the place of grey resources in open archives and institutional repositories, the development of bibliographic control and standardization, and the difficulties of identification and accessing grey documents. The entry ends with some predictions on the future of grey literature and open questions for research in LIS.