Grey literature is material that is not produced by commercial publishers. It is produced by governments, local authorities, health boards, research groups, professional associations, charities, not-for profit organisations, companies etc. It may not be indexed in bibliographic databases and this can make it hard to find. There are many definitions of grey literature, for example:

The term grey literature is generally used to describe materials not published commercially or indexed by major databases.

Olson, C.A. (2015) 'Using the Grey Literature to Enhance Research and Practice in Continuing Education for Health Professionals', Journal of CEHP, 33(1) pp. 1-3.

• Blogs
• Case studies
• Government documents
• Newsletters
• Reports
• Posters 
• Theses

Examples taken from GreyNet International (2021) Available at:  http://greynet.org/greysourceindex/documenttypes.html (Accessed 30th July 2024)>

• The information may only be available as grey literature
• Information is up-to-date
• It can cover special interest or emerging research areas
• It guards against publication bias where only positive results may be commercially published 
• It may give practical examples rather than just theory
• A wide range of opinion is represented, including those who may not use traditional publishing routes

You need to evaluate grey literature as it:

• can vary in quality
• is often not  peer reviewed
• may only be available for a short time
• may be difficult to judge the appropriateness of the methods used
• may be biased