Grey literature is research output produced outside traditional commercial or peer-reviewed academic publishing — reports, theses, preprints, standards, conference proceedings, and government or institutional documents. It is not “lower quality” by definition; it is simply not filtered through a journal’s editorial and peer-review process before release. That distinction is what determines both where you find it and how you should appraise it before citing it.
What counts as grey literature
The operational test is about the publication channel, not the topic or the author’s credentials. A document is grey literature if it was produced by government, academia, business, or industry, in print or electronic form, and is not controlled by commercial publishers — meaning it did not go through a journal’s formal peer-review and editorial production process. A working paper posted to an institutional repository is grey literature even if a Nobel laureate wrote it; the same content, once accepted and typeset by a peer-reviewed journal, is no longer grey literature.
Two edge cases worth naming explicitly:
- Preprints are grey literature. Many preprint servers now run light moderation (screening for plagiarism, completeness, or obvious ethical problems), but that is not peer review, and most preprints are eventually revised before or after formal publication.
- A grey-literature document that later gets peer-reviewed and published stays grey in its original form. If you cite the original report or working paper rather than the peer-reviewed version that superseded it, note that explicitly — a reader checking your citation should not have to guess which version you used.
A typology of grey literature
| Type | Typical producer | Examples | Peer-reviewed? |
|---|---|---|---|
| Preprints | Individual researchers, labs | arXiv, bioRxiv, medRxiv, SSRN, Preprints.org submissions | No (screened, not peer-reviewed) |
| Theses & dissertations | Universities | PhD/master’s theses in an institutional repository | No (examined by committee, not journal peer review) |
| Government & agency reports | National/regional government bodies, NGOs | GAO reports, OECD working papers, WHO technical reports | No (internal review, not journal peer review) |
| Standards & technical specifications | Standards bodies | ISO, NIST, IEEE standards documents | No (consensus/committee process) |
| Conference proceedings & abstracts | Professional societies, conference organizers | Abstracts, posters, extended abstracts not published in a proceedings journal | Usually not (variable, often only abstract-reviewed) |
| Working papers & policy briefs | Research institutes, think tanks, universities | NBER working papers, institutional policy briefs | No |
| Clinical trial registrations & study protocols | Trial sponsors, investigators | ClinicalTrials.gov entries, WHO ICTRP records, published protocols | No (registry entry, not a journal article) |
| Patents | Inventors, assignees, patent offices | USPTO, EPO, WIPO filings | No (examined for patentability, not scientific peer review) |
Where to find grey literature
Grey literature is scattered by design — there is no single index that covers all of it the way a journal database covers peer-reviewed articles. Search across several source types depending on what you need:
- Preprint servers — arXiv (physics, math, CS), bioRxiv and medRxiv (life sciences and health), SSRN (social sciences), ChemRxiv (chemistry).
- Institutional and subject repositories — searchable in aggregate through BASE (Bielefeld Academic Search Engine) and CORE, both of which harvest open-access repositories and preprint servers at scale, and through OpenDOAR, a directory of individual repositories you can search one at a time.
- Trial registries — ClinicalTrials.gov and the WHO International Clinical Trials Registry Platform (ICTRP), which aggregates registries worldwide.
- Government and intergovernmental portals — national government publication portals, the OECD iLibrary, World Bank Open Knowledge Repository, and equivalent agency-specific repositories.
- Standards bodies directly — ISO, national standards bodies (ANSI, BSI), and sector-specific bodies (IEEE, NIST) each maintain their own catalogs.
- Conference and society websites — proceedings and abstracts not indexed elsewhere are often only on the sponsoring society’s own site.
- General academic search engines — Google Scholar indexes a substantial amount of grey literature (theses, reports, preprints) alongside peer-reviewed content, though without the curated inclusion criteria of a subscription database.
For a fuller comparison of how a broad-crawl tool like Google Scholar differs from a curated database in what it includes, see how source type and control over inclusion criteria are compared in Primary vs. Secondary Sources in Research.
Appraising grey literature before you cite it
Because grey literature skips formal peer review, the burden of quality-checking shifts to you as the reader. Work through these questions before relying on a grey-literature source:
- Authority — Who produced it, and what is their institutional standing or track record on this topic? A national statistics agency and an unaffiliated blog are both technically “grey literature,” but they are not equivalent sources.
- Currency — When was it produced, and is there a more recent version, a peer-reviewed follow-up, or a retraction/correction notice?
- Methodological transparency — Does the document describe how data were collected and analyzed clearly enough for you to judge its rigor, or does it just assert conclusions?
- Purpose and funding — Was it produced to inform, or to advocate for a predetermined position (e.g., an industry-funded white paper)? Advocacy purpose doesn’t automatically disqualify a source, but it changes how you should weigh its claims.
- Has it since been superseded? — Search for whether the same work was later published in peer-reviewed form; if so, decide deliberately whether to cite the original grey version, the peer-reviewed version, or both.
This is the same underlying discipline as the CRAAP test (currency, relevance, authority, accuracy, purpose), applied with extra weight on authority and purpose since peer review isn’t available as a quality signal. See the CRAAP Test guide for the full framework applied to any source type.
When you should (and shouldn’t) cite grey literature
Grey literature is not something to avoid on principle — for some questions it is the only source that exists. Government statistics, clinical trial registrations, technical standards, and current policy documents are often published only as grey literature, sometimes years before (or instead of) any peer-reviewed treatment appears.
It is also methodologically required, not optional, in some contexts. The PRISMA 2020 reporting guideline for systematic reviews and meta-analyses explicitly requires reviewers to search trial registries and grey literature sources, not just indexed databases, specifically to reduce the distortion that publication bias introduces when only published, peer-reviewed studies are included.
Where grey literature is weaker is as a substitute for peer-reviewed evidence on a question that peer-reviewed literature has already settled, or as the sole support for a strong causal or clinical claim. Use it where it is the right source for the question — current statistics, official positions, technical specifications, unpublished or in-progress work — and pair it with peer-reviewed evidence, or flag its unreviewed status explicitly, where the claim’s strength depends on it.
Frequently asked questions
Is grey literature reliable?
It varies more than peer-reviewed literature, precisely because it hasn’t been through a standardized editorial filter. A report from a national statistics agency and an unvetted white paper are both grey literature but warrant very different levels of trust. Appraise each source individually rather than treating “grey literature” as a single reliability tier.
Is a preprint grey literature?
Yes. Preprint servers may screen submissions for completeness or obvious problems, but that screening is not peer review, so preprints fall under the grey-literature definition until (and unless) a peer-reviewed version is published.
What’s the opposite of grey literature?
There isn’t a single standard term, but it’s commonly contrasted with “white literature” or simply “peer-reviewed literature” — journal articles, and other outputs that have gone through formal editorial peer review before publication.
Should I cite grey literature in an academic paper?
Often yes, when it’s the right source for the claim — official statistics, standards, registry data, or current policy documents frequently exist only as grey literature. Check your target journal or institution’s citation guidance for how they want grey-literature sources formatted, since standard citation styles don’t always have a dedicated grey-literature template.
Why is it called “grey” literature?
The term reflects its status between two more clearly defined categories: not confidential or classified (“black”), but not formally published through commercial/peer-reviewed channels either (“white”). It sits in the grey area between the two.
