In the English Wikipedia, verifiability means that people are able to check that information comes from a reliable source. Its content is determined by previously published information rather than editors' beliefs, experiences, or previously unpublished ideas or information. Even if you are sure something is true, it must have been previously published in a reliable source before you can add it. If reliable sources disagree with each other, then maintain a neutral point of view and present what the various sources say, giving each side its due weight.

All material in Wikipedia mainspace, including everything in articles, lists, and captions, must be verifiable. Additionally, four types of information must be accompanied by an inline citation to a reliable source that directly supports the material:

• direct quotations,
• material whose verifiability has been challenged,
• material whose verifiability is likely to be challenged, and
• contentious material about living and recently deceased persons.

Any material that needs an inline citation but does not have one may be removed. Please immediately remove contentious material about living people (or existing groups) that is unsourced or poorly sourced.

For how to write citations, see citing sources. Verifiability, no original research, and neutral point of view are Wikipedia's core content policies. They work together to determine content, so editors should understand the key points of all three. Articles must also comply with the copyright policy.

All content must be verifiable. The burden to demonstrate verifiability lies with the editor who adds or restores material, and it is satisfied by providing an inline citation to a reliable source that directly supports the contribution.

Using inline citations, provide reliable, published sources for all:

• direct quotations,
• material whose verifiability has been challenged
• material whose verifiability is likely to be challenged, and
• contentious material about living and recently deceased persons.

The cited source must clearly support the material as presented in the article. Cite the source clearly, ideally giving page number(s)—though sometimes a section, chapter, or other division may be appropriate instead; see Wikipedia:Citing sources for details of how to do this.

Any material lacking an inline citation to a reliable source that directly supports the material may be removed and should not be restored without an inline citation to a reliable source. Whether or how quickly material should be removed for lacking an inline citation to a reliable source depends on the material and the overall state of the article. Consider adding a citation needed tag as an interim step to removing to allow references to be added. When tagging or removing material for lacking an inline citation, state your concern that it may not be possible to find a published reliable source, and the material therefore may not be verifiable. If you think the material is verifiable, you are encouraged to provide an inline citation yourself before removing or tagging it.

Do not leave unsourced or poorly sourced material in an article if it might damage the reputation of living people or existing groups, and do not move it to the talk page. You should also be aware of how Wikipedia:Biographies of living persons also applies to groups.

A cited source on Wikipedia is often a specific portion of text (such as a short article or a page in a book). But when editors discuss sources (for example, to debate their appropriateness or reliability) the word source has four related meanings:

• The work itself (the article, book) and works like it ("An obituary can be a useful biographical source", "A recent source is better than an old one")
• The creator of the work (the writer, journalist: "What do we know about that source's reputation?") and people like them ("A medical researcher is a better source than a journalist for medical claims").
• The publication (for example, the newspaper, journal, magazine: "That source covers the arts.") and publications like them ("A newspaper is not a reliable source for medical claims").
• The publisher of the work (for example, Cambridge University Press: "That source publishes reference works.") and publishers like them ("An academic publisher is a good source of reference works").

All four can affect reliability.

Base articles on reliable, independent, published sources with a reputation for fact-checking and accuracy. Source material must be published, on Wikipedia meaning made available to the public in some form. Unpublished material is not considered reliable. Use sources that directly support the material presented in an article and are appropriate to the claims made. The appropriateness of any source depends on the context. Be especially careful when sourcing content related to living people or medicine.

If available, academic and peer-reviewed publications are usually the most reliable sources on topics such as history, medicine, and science.

Editors may also use material from reliable non-academic sources, particularly if it appears in respected mainstream publications. Other reliable sources include:

• University-level textbooks
• Books published by respected publishing houses
• Mainstream (non-fringe) magazines, including specialty ones
• Reputable newspapers

Editors may also use electronic media, subject to the same criteria (see details in Wikipedia:Identifying reliable sources and Wikipedia:Search engine test).

The best sources have a professional structure for checking or analyzing facts, legal issues, evidence, and arguments. The greater the degree of scrutiny given to these issues, the more reliable the source.

Some newspapers, magazines, and other news organizations host online pages, columns or rolling text they call blogs. These may be acceptable sources if the writers are professionals, but use them with caution because blogs may not be subject to the news organization's normal fact-checking process. If a news organization publishes an opinion piece in a blog, attribute the statement to the writer, e.g. "Jane Smith wrote ..." Never use the blog comments that are left by the readers as sources. For personal or group blogs that are not reliable sources, see § Self-published sources below.

To discuss the reliability of a specific source for a particular statement, consult Wikipedia:Reliable sources/Noticeboard, which seeks to apply this policy to particular cases. For a guideline discussing the reliability of particular types of sources, see Wikipedia:Reliable sources. In the case of inconsistency between this policy and the Wikipedia:Reliable sources guideline, or any other guideline related to sourcing, this policy has priority.

Questionable sources are those that have a poor reputation for checking the facts, lack meaningful editorial oversight, or have an apparent conflict of interest.

Such sources include websites and publications expressing views widely considered by other sources to be promotional, extremist, or relying heavily on unsubstantiated gossip, rumor, or personal opinion. Questionable sources should be used only as sources for material on themselves, such as in articles about themselves; see below. They are not suitable sources for contentious claims about others.

Predatory open access journals are considered questionable due to the absence of quality control in the peer-review process.

Anyone can create a personal web page, self-publish a book, or claim to be an expert. That is why self-published material such as books, patents, newsletters, personal websites, open wikis, personal or group blogs (as distinguished from newsblogs, above), content farms, podcasts, Internet forum postings, and social media postings are largely not acceptable as sources. Self-published expert sources may be considered reliable when produced by an established subject-matter expert, whose work in the relevant field has previously been published by reliable, independent publications. Exercise caution when using such sources: if the information in question is suitable for inclusion, someone else will probably have published it in independent, reliable sources. Never use self-published sources as third-party sources about living people, even if the author is an expert, well-known professional researcher, or writer.

It has been suggested that Wikipedia:Reliable sources#Self-published and questionable sources as sources on themselves and Wikipedia:Biographies of living persons#Using the subject as a self-published source be merged into this section. (Discuss) Proposed since December 2023.

Self-published and questionable sources may be used as sources of information about themselves, usually in articles about themselves or their activities, without the self-published source requirement that they are established experts in the field, so long as:

1. The material is neither unduly self-serving nor an exceptional claim;
2. It does not involve claims about third parties;
3. It does not involve claims about events not directly related to the source;
4. There is no reasonable doubt as to its authenticity; and
5. The article is not based primarily on such sources.

This policy also applies to material made public by the source on social networking websites such as Twitter, Tumblr, LinkedIn, Reddit, and Facebook.

Do not use articles from Wikipedia (whether English Wikipedia or Wikipedias in other languages) as sources, since Wikipedia is a user-generated source. Also, do not use websites mirroring Wikipedia content or publications relying on material from Wikipedia as sources. Content from a Wikipedia article is not considered reliable unless it is backed up by citing reliable sources. Confirm that these sources support the content, then use them directly.

An exception is allowed when Wikipedia itself is being discussed in the article. These may cite an article, guideline, discussion, statistic, or other content from Wikipedia (or a sister project) to support a statement about Wikipedia. Wikipedia or the sister project is a primary source in this case and may be used following the policy for primary sources. Any such use should avoid original research, undue emphasis on Wikipedia's role or views, and inappropriate self-reference. The article text should clarify how the material is sourced from Wikipedia to inform the reader about the potential bias.

Do not reject reliable sources just because they are difficult or costly to access. Some reliable sources are not easily accessible. For example, an online source may require payment, and a print-only source may be available only through libraries. Rare historical sources may even be available only in special museum collections and archives. If you have trouble accessing a source, others may be able to do so on your behalf (see WikiProject Resource Exchange).

Citations to non-English reliable sources are allowed on the English Wikipedia. However, because this project is in English, English-language sources are preferred over non-English ones when they are available and of equal quality and relevance. As with sources in English, if a dispute arises involving a citation to a non-English source, editors may request a quotation of relevant portions of the original source be provided, either in text, in a footnote, or on the article talk page. (See Template:Request quotation.)

If you quote a non-English reliable source (whether in the main text or in a footnote), a translation into English should accompany the quote. Translations published by reliable sources are preferred over translations by Wikipedians, but translations by Wikipedians are preferred over machine translations. When using a machine translation of source material, editors should be reasonably certain that the translation is accurate and the source is appropriate. Editors should not rely upon machine translations of non-English sources in contentious articles or biographies of living people. If needed, ask an editor who can translate it for you.

The original text is usually included with the translated text in articles when translated by Wikipedians, and the translating editor is usually not cited. When quoting any material, whether in English or in some other language, be careful not to violate copyright; see the fair-use guideline.

While information must be verifiable for inclusion in an article, not all verifiable information must be included. Consensus may determine that certain information does not improve an article, and other policies may indicate that the material is inappropriate. Such information should be omitted or presented instead in a different article. The responsibility for achieving consensus for inclusion is on those seeking to include disputed content.

If you want to request an inline citation for an unsourced statement, you can tag a sentence with the {{citation needed}} template by writing {{cn}} or {{fact}}. Other templates exist for tagging sections or entire articles here. You can also leave a note on the talk page asking for a source, or move the material to the talk page and ask for a source there. To request verification that a reference supports the text, tag it with {{verification needed}}. Material that fails verification may be tagged with {{failed verification}} or removed. It helps other editors to explain your rationale for using templates to tag material in the template, edit summary, or on the talk page.

Take special care with contentious material about living and recently deceased people. Unsourced or poorly sourced material that is contentious, especially text that is negative, derogatory, or potentially damaging, should be removed immediately rather than tagged or moved to the talk page.

Any exceptional claim requires multiple high-quality sources. Warnings (red flags) that should prompt extra caution include:

• Surprising or apparently important claims not covered by multiple mainstream sources;
• Challenged claims that are supported purely by primary or self-published sources or those with an apparent conflict of interest;
• Reports of a statement by someone that seems out of character or against an interest they had previously defended;
• Claims contradicted by the prevailing view within the relevant community or that would significantly alter mainstream assumptions—especially in science, medicine, history, politics, and biographies of living and recently dead people. This is especially true when proponents say there is a conspiracy to silence them.

Do not plagiarize or breach copyright when using sources. Summarize source material in your own words as much as possible; when quoting or closely paraphrasing a source, use an inline citation, and in-text attribution where appropriate.

Do not link to any source that violates the copyrights of others per contributors' rights and obligations. You can link to websites that display copyrighted works as long as the website has licensed the work or uses the work in a way compliant with fair use. Knowingly directing others to material that violates copyright may be considered contributory copyright infringement. If there is reason to think a source violates copyright, do not cite it. This is particularly relevant when linking to sites such as Scribd or YouTube, where due care should be taken to avoid linking to material violating copyright.

Even when information is cited to reliable sources, you must present it with a neutral point of view (NPOV). Articles should be based on thorough research of sources. All articles must adhere to NPOV, fairly representing all majority and significant-minority viewpoints published by reliable sources, in rough proportion to the prominence of each view. Tiny-minority views need not be included, except in articles devoted to them. If there is a disagreement between sources, use in-text attribution: "John Smith argues X, while Paul Jones maintains Y," followed by an inline citation. Sources themselves do not need to maintain a neutral point of view. Indeed, many reliable sources are not neutral. Our job as editors is simply to summarize what reliable sources say.

If no reliable, independent sources can be found on a topic, Wikipedia should not have an article on it (i.e., the topic is not notable). However, notability is based on the existence of suitable sources, not on the state of sourcing in an article (WP:NEXIST).

The no original research policy (NOR) is closely related to the Verifiability policy. Among its requirements are:

1. All material in Wikipedia articles must be attributable to a reliable published source. This means a reliable published source must exist for it, whether or not it is cited in the article.
2. Sources must support the material clearly and directly: drawing inferences from multiple sources to advance a novel position is prohibited by the NOR policy.
3. Base articles largely on reliable secondary sources. While primary sources are appropriate in some cases, relying on them can be problematic. For more information, see the Primary, secondary, and tertiary sources section of the NOR policy, and the Misuse of primary sources section of the BLP policy.

• Reliable sources
• Identifying reliable sources (medicine)

• Backlog – links to articles that need citations added
• Template index/Sources of articles – maintenance templates for articles with sourcing problems
• The Wikipedia Library – free access to newspapers, journals, and magazines for experienced editors
• WikiProject Resource Exchange – where you can ask for help with checking an individual source

• Wales, Jimmy. "Insist on sources", WikiEN-l, July 19, 2006: "I really want to encourage a much stronger culture which says: it is better to have no information, than to have information like this, with no sources."—referring to a rather unlikely statement about the founders of Google throwing pies at each other.