The threshold for inclusion in Wikipedia is verifiability, not truth. "Verifiable" in this context means that any reader should be able to check that material added to Wikipedia has already been published by a reliable source. Editors should provide a reliable source for quotations and for any material that is challenged or is likely to be challenged, or it may be removed.

Wikipedia:Verifiability is one of Wikipedia's core content policies. The others include Wikipedia:No original research and Wikipedia:Neutral point of view. Jointly, these policies determine the type and quality of material that is acceptable in Wikipedia articles. They should not be interpreted in isolation from one another, and editors should try to familiarize themselves with all three.

For how to write citations, see Wikipedia:Citing sources

The burden of evidence lies with the editor who adds or restores material. Material that is challenged or likely to be challenged needs a reliable source, which should be cited in the article. Quotations should also be attributed. If an article topic has no reliable, third-party sources, Wikipedia should not have an article on it.

Any edit lacking a source may be removed, but editors may object if you remove material without giving them a chance to provide references. If you want to request a source for an unsourced statement, consider moving it to the talk page. Alternatively, you may tag the sentence by adding the {{fact}} template, or tag the article by adding {{Not verified}} or {{Unreferenced}}. Leave an invisible HTML comment, a note on the talk page, or an edit summary explaining what you have done.^[1]

Be careful not to go too far on the side of not upsetting editors by leaving unsourced information in articles for too long, or at all in the case of information about living people. Jimmy Wales, founder of Wikipedia, has said of this: "I can NOT emphasize this enough. There seems to be a terrible bias among some editors that some sort of random speculative 'I heard it somewhere' pseudo information is to be tagged with a 'needs a cite' tag. Wrong. It should be removed, aggressively, unless it can be sourced. This is true of all information, but it is particularly true of negative information about living persons."^[2]

See also: Wikipedia:Reliable sources and Wikipedia:Biographies of living persons

Articles should rely on reliable, third-party published sources with a reputation for fact-checking and accuracy. Sources should be appropriate to the claims made: exceptional claims require exceptional sources.

All articles must adhere to Wikipedia's neutrality policy, with their writing fairly representing all significant viewpoints that have been published by reliable published sources, in proportion to the prominence of each view.^[3]

Articles should be referenced to reliable published sources that have a reputation for accuracy. Sources should be appropriate for the topic and the claims made, with appropriateness determined by editorial judgment and common sense, guided by consensus. In general, the most reliable sources are peer-reviewed journals and books published in university presses; university-level textbooks; magazines, journals, and books published by respected publishing houses; and mainstream newspapers. As a rule of thumb, the greater the degree of scrutiny involved in checking facts, analyzing legal issues, and scrutinizing the evidence and arguments of a particular work, the more reliable it is.

Academic and peer-reviewed sources are highly valued and should usually be given most weight in areas such as history, medicine, and science. Material from reliable non-academic sources can also be used in these areas, if they are respected mainstream sources with good reputations for accuracy. The appropriateness of sources always depends on context and non-academic sources should not be used as the sole support for bizarre or extraordinary claims.

Sources of questionable reliability

In general, sources of questionable reliability are sources with a poor reputation for fact-checking or with no fact-checking facilities or editorial oversight. Sources of questionable reliability should only be used in articles about themselves. (See below.) Articles about such sources should not repeat any potentially libelous claims the source has made about third parties, unless those claims have also been published by reliable sources.

Self-published sources (online and paper)

Anyone can create a website or pay to have a book published, then claim to be an expert in a certain field. For that reason, self-published books, personal websites, and blogs are largely not acceptable as sources.^[4]

Self-published material may be acceptable when produced by a well-known, professional researcher in a relevant field. These may be acceptable so long as their work has been previously published by reliable third-party publications. However, exercise caution: if the information in question is really worth reporting, someone else is likely to have done so.

Self-published sources should never be used as third-party sources about living persons, even if the author is a well-known professional researcher or writer; see WP:BLP.

Self-published and questionable sources in articles about themselves

Material from self-published sources and sources of questionable reliability may be used in articles about themselves, so long as:

• it is relevant to their notability;
• it is not contentious;
• it is not unduly self-serving;
• it does not involve claims about third parties;
• it does not involve claims about events not directly related to the subject;
• there is no reasonable doubt as to who wrote it.

Because this is the English Wikipedia, for the convenience of our readers, English-language sources should be used in preference to foreign-language sources, assuming the availability of an English-language source of equal quality, so that readers can easily verify that the source material has been used correctly.

Keep in mind that translations are subject to error, whether performed by a Wikipedia editor or a professional, published translator. In principle, readers should have the opportunity to verify for themselves what the original material actually said, that it was published by a credible source, and that it was translated correctly.

Therefore, when the original material is in a language other than English:

• Where sources are directly quoted, published translations are generally preferred over editors performing their own translations directly.
• Where editors use their own English translation of a non-English source as a quote in an article, there should be clear citation of the foreign-language original, so that readers can check what the original source said and the accuracy of the translation.

• WikiProject Fact and Reference Check
• Forum for Encyclopedic Standards
• WikiProject Resource Exchange

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• Jimmy Wales. "WikiEN-l insist on sources", WikiEN-l mailing list, July 19, 2006.