What is “Grey Literature” (aka: Gray Literature – alternative spelling)?
Grey literature involves:
“manifold document types produced on all levels of government, academics, business and industry in print and electronic formats that are protected by intellectual property rights, of sufficient quality to be collected and preserved by libraries and institutional repositories, but not controlled by commercial publishers; i.e. where publishing is not the primary activity of the producing body”. Prague Definition (2010).
The Public Health LKS website also provides a list of sources.
Dean Giustini has produced an HLWiki page, which is rated by some experts to be very useful.