What is grey literature?
The Fourth International Conference on Grey Literature (GL '99) in Washington, DC, in October 1999 defined grey literature as follows: "That which is produced on all levels of government, academics, business and industry in print and electronic formats, but which is not controlled by commercial publishers."
Why is it important?
Grey literature is important because everything is not available in scholarly articles. It can help you find evidence-based material that may not be available in scholarly, peer-reviewed journals.
If you're looking in a database that has scholarly articles, see if there is an option to select Conference Proceedings, Government Documents, Dissertations/Thesis, or Reports. Your options will vary between databases.
For example:
bioRxiv
"a free online archive and distribution service for unpublished preprints in the life sciences." a
Grey Guide MedRxiv
Preprint server for the health sciences
New York Academy of Medicine: Grey Literature Report
Public Health and Health Policy Resources
OpenGrey
European database for grey literature covering a variety of disciplines.
SciTech Connect
"SciTech Connect is a portal to free, publicly available DOE -sponsored R&D results including technical results, bibliographic citations, journal articles, conference papers, books, multimedia software, and data information.
WorldWideScience
Searches the deep web through national and international scientific databases and portals