Skip to Main Content
It looks like you're using Internet Explorer 11 or older. This website works best with modern browsers such as the latest versions of Chrome, Firefox, Safari, and Edge. If you continue with this browser, you may see unexpected results.

Systematic Reviews: Grey Literature

What is Grey Literature?

“Variously defined as unconventional literature, literature not available via traditional publishing, and publications with little or no general distribution, grey literature represents an immense stash of information that is crucial to various researchers and information professionals.”

Auger, Charles P. Information Sources in Grey Literature. London: Bowker-Saur, 1998.

Searching the grey literature may seem challenging as there are so many sources. You should search the resources that are most relevant to your systematic review.


See also  this free online resource for grey literature searching, compiled by CADTH, the Canadian Agency for Drugs and Technologies in Health: Grey Matters: a practical tool for searching health-related grey literature

Searching for Grey Literature

Some of our databases, depending on the discipline, index grey literature as well as journals.

Check the database scope note.

Essential Sources:


Trials Registers


EU Clinical Trials Register

UK Clinical Trials Gateway

Development of a single international trials register at ClinicalTrials.Gov

ICTRP International Clinical Trials Registry Platform

See Medical and health-related trials registers and research registers 

This resource provides a listing of trials and research registers and a quick reference guide to the search basics for each resource. If you would like to suggest registers to add to this resource please send the details to Julie Glanville at

In addition see Cochrane Handbook Chapter Six: 6.2.3 Unpublished and ongoing studies:

National and international trials registers

Subject-specific trials registers

Pharmaceutical industry trials registers

Trials results registers and other sources

See also:

Cochrane Handbook Chapter Ten:  10.3.2  Including unpublished studies in systematic reviews:

 Including unpublished studies in systematic reviews

Higgins, Julian., Green, Sally  and Cochrane Collaboration. Cochrane Handbook for Systematic Reviews of Interventions. Chichester, England: Wiley-Blackwell, 2011 


Conference Proceedings

Theses & Dissertations

See our Theses and dissertations webpage.

See also the full list of national and international theses databases to which we provide access.

Guidance from Cochrane handbook


See 4.S1 Technical Supplement to Chapter 4: Searching for and selecting studies Section 1.1.6 Grey literature databases:


Guidance from Centre for Reviews & Dissemination, University of York

In addition to searching electronic databases, the Centre for Reviews and Dissemination at University of York recommend locating studies by:

 • Visually scanning reference lists from relevant studies

 • Handsearching key journals and conference proceedings

 • Contacting study authors, experts, manufacturers, and other organisations

 • Searching relevant Internet resources

Internet searching can be a useful means of retrieving grey literature, such as unpublished papers, reports and conference abstracts. Identifying and scanning specifi c relevant websites will usually be more practical than using a general search engine such as ‘Google’. Reviews of transport and ‘welfare to work’ programmes have reported how Internet searching of potentially relevant websites was effective in identifying additional studies to those retrieved from databases.40, 41 It is worth considering using the Internet when investigating a topic area where it is likely that studies have been published informally rather than in a journal indexed in a bibliographic database. Internet searching should be carried out in as structured a way as possible and the procedure documented (see Appendix 3).

 • Citation searching  See also

Citation searching involves selecting a number of key papers already identified for inclusion in the review and then searching for articles that have cited these papers. This approach should identify a cluster of related, and therefore highly relevant, papers

 Citation searching used to be limited to using Web of Science and Scopus, but other resources (including CINAHL, PsycINFO, and Google Scholar) now include cited references in their records so these are also available for citation searching. Using similar services offered by journals such as the BMJ can also be helpful.


• Using a project Internet site to canvas for studies

 University of York. NHS Centre for Reviews & Dissemination. Systematic Reviews : CRD's Guidance for Undertaking Reviews in Healthcare. York: Centre for Reviews & Dissemination, 2009.