The term 'grey literature' is used to describe materials not published commercially or indexed by major databases. While GL may be of questionable quality it has been shown to have an impact in research, teaching and learning. Sometimes, GL is the only source of information for specific research questions. GL may not go through a peer-review process, and its authority must be scrutinized.
There is now a separate guide for grey literature in health which has information on general sources, conference proceedings, institutional repositories, clinical trials, guidelines and statistics.