Grey literature is generally material not published commercially or indexed by major databases. 

A more complete definition is information "produced on all levels of government, academics, business and industry in electronic and print formats not controlled by commercial publishing i.e. where publishing is not the primary activity of the producing body." (http://www.greynet.org/)

Reports, preprints, working documents, research papers, theses and dissertations, clinical trials, bibliographies, newsletters, patents, statistical documents, white papers, pamphlets, informal communication (e.g., blogs, podcasts, email), and more.

• Deep Web Searching
• Institutional Repositories
• Reports and Research, Theses/Dissertations, Clinical Trials

(Only SOME of the Invisible Web [grey box] material is available via UW Libraries databases and sites listed on this guide.)
Data from P. Gil, "What is the invisible web?" Dec. 2010. Schematic by N. Tann, Goshen College, March 2011.

Credits: The Harold & Wilma Good Library

The Deep Web, also known as the Invisible Web, is a portion of the web not reached by standard search engines, such as Google and Bing. Less than 10% of the web is indexed by search engines, with the remaining 90% of web content called the Deep Web. It is estimated to be 2-500x bigger than the visible web. 

Here are some general resources to search the Deep Web:

• Grey Matters: A practical search tool for evidence-based medicine
• Kennedy School of Government Think Tank Search a custom Google search of over 670 think tank and research center websites
• MedNar:  deep web search engine of 60+ authoritative medical collections (Article on using MedNar)
• OAIster:  digital resources from open archive collections worldwide 
• OpenGrey:  an archive of grey literature from Europe, downloadable as MySQL
• WorldWideScience.org:  global science gateway to international scientific databases
• WWW Virtual Library:  high quality subject guides
• OpenMD: search reference sites, government agencies, health organizations, and journals.
• BASE Search: for academic web resources

An Institutional Repository is a virtual space where a university or research institute collects and preserves its research and findings. Information in Repositories is considered grey literature since these resources are not traditionally published.

• ResearchWorks at the University of Washington: digital repository of articles, technical reports, dissertations, datasets, images and other file types produced at UW by faculty and researchers.
• OpenDOAR: directory of open access repositories

Reports and Research:

• NIH RePORTER: Describes research projects funded by NIH and other DHHS agencies.
• NLM Gateway: Find conference abstracts and research projects
• Sigma Nursing Repository (formerly known as the Virginia Henderson International Nursing Library)
• Virtual Technical Reports Center: List of institutions with links to technical reports, preprints, eprints, reprints, dissertations, theses, and research reports of all kinds

Theses/Dissertations

• ProQuest Dissertations & Theses Full-text comprehensive database collection of dissertations and theses
• EThOS: Access to UK theses
• Open Access Theses and Dissertations (OATD) Full text collection of open access graduate theses and dissertations published around the world

Clinical Trials

• CenterWatch: Global source for clinical trials information, offering news, analysis, study grants, career opportunities, and trial listings to professionals and patients
• ClinicalTrials.gov: Registry and results database of federally and privately supported clinical trials conducted in the United States and around the world
• EU Clinical Trials Register: Registered clinical trials in the European Union 
• ISRCTN Register: Registered randomized controlled trials and other studies to assess the efficacy of health care interventions around the world