"Grey literature" (or "gray literature") is a term used to describe a variety of materials that fall outside the traditional publishing and distribution channels.
Grey literature can be produced by government agencies, non-profit organizations, academic institutions and departments, and private companies.
These materials do not go through the same rigorous peer-review process that most scholarly journal articles do, but they can still provide valuable information to supplement peer-reviewed resources. Be mindful to evaluate these sources carefully and only use them when appropriate.
Examples of grey literature include:
Professional associations are organizations dedicated to individuals in a specific field. These organizations often focus on professional development opportunities like workshops, conferences, and networking. They can also be good sources of historical and/or current information in the field; some associations formally publish reports or conference proceedings, while others host newsletters or blogs. These kinds of publications are considered grey literature.