Material that is actually challenged by another editor requires a source or it may be removed; and anything likely to incur a reasonable challenge should be sourced to avoid disputes and to aid readers (see WP:BURDEN). In practice, this means most such statements are backed by an inline citation. In case of multiple possible references for a statement, the best reliable sources should be used.
Quotations: Add an inline citation when quoting published material, whether within quotation marks or not, whether using direct or indirect speech. When using footnotes, the citation should be placed in the first footnote after the quotation. In-text attribution is often appropriate.
Close paraphrasing: Add an inline citation when closely paraphrasing a source's words. In-text attribution is often appropriate, especially for statements describing a person's published opinions or words. In-text attribution is not appropriate for other forms of close paraphrasing, such as if you paraphrase "The sky is usually blue" as "The sky is often the color blue".
Contentious statements about living people: Editors must take particular care adding biographical material about a living person to any Wikipedia page. Such material requires a high degree of sensitivity; do not leave unsourced information that may damage the reputation of living persons or organizations in articles.
Exceptional claims: Exceptional claims in Wikipedia require high-quality reliable sources (see WP:REDFLAG):
Surprising or apparently important claims not covered by mainstream sources;
Reports of a statement by someone that seems out of character, embarrassing, controversial, or against an interest they had previously defended;
Claims that are contradicted by the prevailing view within the relevant community, or which would significantly alter mainstream assumptions, especially in science, medicine, history, politics, and biographies of living persons, and especially when proponents consider that there is a conspiracy to silence them.
Other: Opinions, data and statistics, and statements based on someone's scientific work should be cited and attributed to their authors in the text.
General common knowledge: Statements that the average adult recognizes as true. Examples: "Paris is the capital of France" or "Humans normally have two arms and two legs."
Subject-specific common knowledge: Material that someone familiar with a topic, including laypersons, recognizes as true. Example (from Processor): "In a computer, the processor is the component that executes instructions."
Plot of the subject of the article: If the subject of the article is a book or film or other artistic work, it is unnecessary to cite a source in describing events or other details. It should be obvious to potential readers that the subject of the article is the source of the information. If the subject of the article is a work that has been published or broadcast in a serial manner, then citing the episode, issue or book can aid comprehension for readers not familiar with the whole of the serial work. It also aids verification if editors are concerned about inappropriate use of the artistic work (a primary source) for interpretation.
Cited elsewhere in the article: If the article mentions the fact repeatedly, it suffices to cite it once. Uncontroversial content in the lead is often not cited, as it is a generalization of the cited body text. Subleads (generalized opening statements summarizing specific sections, paragraphs, etc.) may also be verified by the citations of the following text. It is permissible to cite such content (including with <ref>Sublead generalization supported by all the citations in this section</ref>), but not mandatory.
Because the lead will usually repeat information also in the body, editors should balance the desire to avoid redundant citations in the lead with the desire to aid readers in locating sources. Leads are usually written at a greater level of generality than the body, and information in the lead section of non-controversial subjects is less likely to be challenged and less likely to require a source. There is not, however, an exception to citation requirements specific to leads. Complex, current, or controversial subjects may require many citations; others, few or none. Contentious material about living persons must always be cited, regardless of the level of generality.
The distance between material and its source is a matter of editorial judgment. The source of the material should always be clear, and editors should exercise caution when rearranging cited material to ensure that the text–source relationship isn't broken.
If you write a multi-sentence paragraph that draws on material from one source, the source need not be cited after every single sentence unless the material is particularly contentious. When multiple sources are used within a paragraph, these can be bundled into a single footnote if desired, so long as the footnote makes clear which source supports which points in the text.
The right to challenge: Any editor has the right to challenge unsourced material by opening a discussion on the talk page or by tagging it. Material that should be removed without discussion includes unsourced contentious material about a living person, clear examples of original research, and anything that is ludicrous or damaging to the project.
Challenges should not be frivolous: Challenges should not be made frivolously or casually, and should never be made to be disruptive or to make a point. Editors making a challenge should have reason to believe the material is contentious, false, or otherwise inappropriate.
Responses must be forthcoming: Editors who wish to respond to the challenge should do so in a timely manner. If no response is forthcoming, the challenger may tag or remove the statement in question. Unless the material falls into the class that should be removed without discussion, the challenger should await a timely response prior to removing.