For public health issues and policy studies, grey literature can be a supplement for traditional peer reviewed research. A technical definition of grey literature comes from the ICGL in Prague (2010):
Grey literature stands for manifold document types produced on all types 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,f but not controlled by commercial publishers , i.e., where publishing is not the primary activity of the producing body.
A more practical, less "academic" definition is the corpus of high quality information that is created outside the traditional scholarly communications infrastructure of journals and publishers.
Examples of grey literature: