Web addresses break. A journal moves to a new publisher, a site is rebuilt, and the link you copied two years ago leads to an error page. A reference that depends on that link no longer lets a reader find what you read.
The DOI was made to solve this. It is a code that belongs to the work itself, not to the place where the work is kept today. The main citation styles all ask for it when a work has one.
This guide covers what a DOI is, where to find one, how to run a DOI lookup when it is missing, how each style wants it written, and what to do when it does not work. Once you have the DOI, EdCitation's free Cite a source turns it into the finished reference in your style, read from the record the DOI points to instead of typed out by hand.
What is a DOI?
A DOI (digital object identifier) is a permanent, unique code given to a published work, which always leads to that work's current location online. The DOI Foundation (n.d.-b), the not-for-profit body that runs the system, describes a DOI as "a digital identifier of an object, any object", physical, digital or abstract. The system is defined by an international standard, ISO 26324, approved in November 2010, published in May 2012 and revised in 2022 (DOI Foundation, n.d.-b), with a third edition in March 2025 (International Organization for Standardization, 2025).
Publishers do not mint DOIs alone. They register them through a registration agency and undertake to keep the record up to date when the work moves. The agency most students meet is Crossref, which registers DOIs for journal articles, books, chapters and conference papers. DataCite registers them for research data, software and other research outputs. The DOI Foundation (n.d.-a) lists the others, including agencies for China, Japan, Korea and the European Union's publications.
The two parts of a DOI
Every DOI has two parts, separated by a forward slash. Take 10.1086/725865:
| Part | In the example | What it is |
|---|---|---|
| Prefix | 10.1086 | Identifies who registered the DOI. It always begins with "10." and is followed by a number given to the publisher or organisation. |
| Suffix | 725865 | Chosen by the publisher to identify this one work. It can contain letters, numbers and a few punctuation marks. |
To use a DOI, put it after https://doi.org/ in a browser. The address https://doi.org/10.1086/725865 opens the article on its publisher's site, wherever that now is.
What a DOI promises, and what it does not
Crossref (n.d.-a) sets out the rules its members work to, and three of them matter to anyone writing a reference. DOI suffixes are case insensitive, so 10.1006/abc and 10.1006/ABC reach the same work. Suffixes use only Roman letters, digits and a short list of punctuation marks, which is why a reference containing an en dash inside a DOI has been mangled somewhere. And a DOI is not meant to be deleted or changed once it is registered: when a journal moves to another publisher, the new owner keeps the same DOI and updates where it points.
What a DOI does not promise is quality. It records that a work was registered, not that it was peer reviewed, and registration agencies issue DOIs for preprints, datasets, reports and theses as well as for journal articles. A DOI that resolves tells you the work exists. It does not tell you the work is good, or that it has not been retracted.
Where do I find the DOI on an article?
The DOI is usually printed on the first page of the article, in the header, the footer or beside the copyright line, and it is shown on the article's page on the journal's website, near the title or the citation details. It always starts with "10.".
If it is not there, look in these places:
- The database record. In a library database, the record for the article normally lists the DOI.
- The address bar. Many publishers build the DOI into the article's web address, starting at "10.".
- Other papers' reference lists. A later paper that cites your article will often give its DOI.
- The PDF's properties. Some publishers put the DOI in the file's metadata, which your PDF reader will show.
Copy and paste the DOI. Do not retype it. A suffix can mix the letter O with the number 0, and one wrong character means a link that fails.
How do I find a DOI that is missing?
Search for the article's title in Crossref's free search at search.crossref.org, and check that the result matches your article before you copy the DOI.
A DOI lookup in three steps
- Go to search.crossref.org and paste the full title of the article, in quotation marks if the title is short or common.
- Find the matching result and compare the authors, the journal, the year and the page numbers with the article in front of you. A near-match is not a match: two papers can share a title.
- Copy the DOI shown under the result, and open https://doi.org/ plus that DOI to confirm it lands on the right article.
If the DOI is only a means to the reference, skip the copying: paste the title into Cite a source on EdCitation, which accepts a title as readily as a DOI and looks the work up in the publisher's record. Step 2 still applies to what comes back. Two papers can share a title, so hold the authors, journal and year of the entry against the article in front of you before you use it.
For a whole reference list, Crossref (n.d.-d) also runs Simple Text Query, which takes up to 1,000 references in one submission, one to a line, and returns the DOIs it can match. It can return PubMed IDs alongside them, which is useful in the health sciences. If the question about your list is whether every entry is real rather than which DOI each one has, paste it into Verify references instead, and each entry comes back verified, marked "check this", or not found.
Some real sources have no DOI at all. Older articles, many books, most web pages, newspaper articles and a large share of government and charity reports have none. That is not a fault in your reference. Each style says what to give in its place, which is usually a URL for something read online, or nothing extra for a printed work.
How do I write a DOI in APA, MLA, Chicago and Harvard?
All four write the DOI as a full link beginning with https://doi.org/ and place it at the end of the reference. They differ in the punctuation around it.
| Style | How the DOI is written | End of the entry |
|---|---|---|
| APA 7 | https://doi.org/10.1086/725865 | No full stop after the DOI |
| MLA 9 | https://doi.org/10.1086/725865. | Full stop. MLA prefers a DOI to a permalink or a URL |
| Chicago 18 | https://doi.org/10.1086/725865. | Full stop. Chicago prefers a link based on a DOI for a journal article read online |
| Harvard (Cite Them Right) | Available at: https://doi.org/10.1086/725865 | No access date when a DOI is used |
Three of those rules are worth having in the words of the body that made them.
The American Psychological Association (2026) tells writers to include a DOI for all works that have one, and not to put a full stop after it. The same guide says not to add a URL or database information for a work found in an academic research database, and to give a URL only for an ebook from another kind of website. So a journal article you read in a library database ends at its DOI, with nothing about the database.
The Modern Language Association (2016) sets out an order of preference for the location element of an online work: the DOI first, then a permalink, then a URL. The University of Westminster (n.d.), whose guide follows the 13th edition of Cite Them Right, is blunt about the Harvard rule: do not include an accessed date when you are using a DOI. An access date belongs with an ordinary web address, which can change without notice.
The older forms you will still see
Crossref's display guidelines have asked members since March 2017 to show DOIs as full https://doi.org/ links, with no "doi:" label and no "dx." in the address. Papers written before that, and guides written for older editions, show three forms you should convert rather than copy:
doi:10.1086/725865becomes https://doi.org/10.1086/725865.http://dx.doi.org/10.1086/725865becomes https://doi.org/10.1086/725865. The old address still works, but it is no longer what publishers are asked to print.- "Retrieved from https://doi.org/..." loses "Retrieved from", which APA 7 dropped.
What is the difference between a DOI, a URL, an ISBN and a PMID?
A DOI identifies a work permanently, a URL gives one current location of it, an ISBN identifies one edition of a book, and a PMID identifies an article's record in PubMed. Only the DOI and the URL are normally written in a student's reference.
| Identifier | What it identifies | Looks like | Goes in the reference? | Cite a source takes it? |
|---|---|---|---|---|
| DOI | One work, permanently | 10.1086/725865 | Yes, whenever the work has one | Yes |
| URL | A location on the web, which can change | https://www.who.int/news-room/fact-sheets | Yes, for online works with no DOI | Yes |
| ISBN | One edition and format of a book | 13 digits | No. APA's own book examples end at the publisher and the DOI or URL, with no ISBN, and the other main styles do not ask for one | Yes, looked up in Open Library |
| PMID | One citation record in the PubMed database | A number with no letters | Not in APA, MLA, Chicago or Harvard | Yes |
An ISBN and a PMID are still the quickest way to look a source up exactly, which is why reference tools accept them even though they do not print them. EdCitation takes all four. Given an ISBN, it finds the book in Open Library and takes the authors from that edition's own record, which is why the ISBN on the edition you actually read is the one to give it.
What does it mean if a DOI does not work?
A DOI that leads to a "DOI not found" page has almost always been copied wrongly, or was printed before the publisher registered it. Crossref (n.d.-c) lists the causes it sees in its own resolution logs: a DOI that was never registered, a DOI published ahead of its registration, a DOI displayed incorrectly somewhere else with a digit missing or a full stop from the end of a sentence stuck to it, typing errors that confuse O with 0 or l with 1, problems with URL encoding, and bot activity, which Crossref notes includes errors made by large language models.
So check the copy first. Find the article by its title, compare the DOI on the publisher's page with the one you have, character by character, and watch the end of the string for a stray full stop.
When the DOI works but opens the wrong paper
This needs more care than a broken link. If the DOI resolves but the title and authors on the page do not match the reference, the reference is wrong, and it may describe a paper that was never written. Attaching a real DOI to an invented title is a known pattern in references produced by AI tools, and Crossref's own note on large language models suggests the traffic reaches its servers. A fast first test is to paste the reference into EdCitation's Verify references: it tells you whether the entry is verified, doubtful (the label reads "check this") or not found, and a reference it could not check is never passed off as missing. What to do after that is set out in How to check whether a reference is real.
How do I turn a DOI into a finished reference?
Give the DOI to a tool that looks the work up in the publisher's record and arranges the details in your style, then read the result against the article. The record behind a DOI holds the authors, title, journal, volume, issue, pages and year, which is every element a reference needs.
Cite a source on EdCitation is the best tool for that job, for the reason this guide keeps coming back to: it looks the reference up and never writes it. Crossref (n.d.-c) counts errors made by large language models among the causes of DOIs that fail to resolve, and a chatbot composing a reference from what it remembers can hand you exactly that fault. No chatbot reads the record behind a DOI and arranges it for you the way Cite a source does. Paste a DOI, or a title, a PubMed ID, an ISBN or a web address if you have no DOI, and it builds the reference from that record in any of six styles, from APA 7 and MLA 9 to Chicago 18 author-date, Cite Them Right Harvard, IEEE and Vancouver, so every element is the publisher's and none is retyped. It also screens the source for retraction, which a working DOI, as the first section showed, cannot tell you. The style rules it follows are set out on the APA style page.
The one thing to read by hand
The title's capitals. APA 7 and Vancouver want an article title in sentence case, and Cite a source sets a title registered in title case that way, keeping names, places and initialisms. Glance at any name made of ordinary words, which can be lowered with the rest, and at a title the publisher registered with capitals on only a few words, which is kept as registered. The other elements arrive as the publisher recorded them, which is the point; your part is to confirm the record is the work you read.
What it costs
Cite a source costs nothing and asks for no sign-up, and nor does Verify references; a free account adds saved reference lists. For a finished paper, Pro ($8 a month) brings References from a file: upload the document, and each reference in it is checked while each citation in the text is paired with its entry. Max ($24 a month) brings Theoretics QA and the Library. Both plans, and the 25% saving for a year at once, are on Pricing.
The same article, cited in full in each of the main styles, is in APA vs MLA vs Chicago.
Quick questions
Is a DOI the same as a URL?
No. A DOI is a code that identifies the work, and a URL is an address for one place where the work can be found. A DOI can be written as a URL by putting https://doi.org/ in front of it, and that link keeps working when the publisher's own address changes.
Do all journal articles have a DOI?
No. Most recent articles from established journals have one, but some older articles, some small journals, and many reports and web pages do not. If a work has no DOI, follow your style's rule for a URL or for a print source.
Are DOIs case sensitive?
No. Crossref tells its members that DOI suffixes are case insensitive, so 10.1006/abc and 10.1006/ABC lead to the same work. Lower case is preferred in print because it is easier to read and to copy accurately.
Do I need a DOI if I read the article in print?
In APA 7, yes. APA asks for the DOI whenever the work has one, whether you used the online or the print version. MLA and Chicago ask for it for works read online.
Should I write doi: or https://doi.org/ in my reference?
Write https://doi.org/ followed by the DOI. APA 7, MLA 9, Chicago 18 and Cite Them Right Harvard all use the full link form, and Crossref has asked publishers to display it that way since March 2017. The "doi:" label belongs to older editions.
Can I turn a DOI straight into a reference?
Yes. Paste it into EdCitation's free Cite a source, pick the style, and the entry is built from the record the DOI resolves to. For APA 7 and Vancouver it sets a title deposited in title case in sentence case and keeps names and initialisms; glance at any name made of ordinary words.
References
- American Psychological Association. (2026). Reference guide for journal articles, books, and edited book chapters. APA Style. https://apastyle.apa.org/instructional-aids/reference-guide.pdf
- Crossref. (n.d.-a). Constructing your DOIs. Crossref. https://www.crossref.org/documentation/member-setup/constructing-your-dois/
- Crossref. (n.d.-b). Crossref DOI display guidelines. Crossref. https://www.crossref.org/display-guidelines/
- Crossref. (n.d.-c). Resolution report. Crossref. https://www.crossref.org/documentation/reports/resolution-report/
- Crossref. (n.d.-d). Simple Text Query. Crossref. https://www.crossref.org/documentation/retrieve-metadata/simple-text-query/
- DOI Foundation. (n.d.-a). Existing registration agencies. DOI Foundation. https://www.doi.org/the-community/existing-registration-agencies/
- DOI Foundation. (n.d.-b). What is a DOI? DOI Foundation. https://www.doi.org/the-identifier/what-is-a-doi/
- International Organization for Standardization. (2025). Information and documentation: Digital object identifier system (ISO Standard No. 26324:2025). https://www.iso.org/standard/88862.html
- Modern Language Association. (2016, August 11). I am citing a paginated journal article that appears online. Should I include the page range, the URL, or the DOI as the location element in the works-cited-list entry? MLA Style Center. https://style.mla.org/page-range-and-doi/
- University of Chicago Press. (n.d.). Author-date: Sample citations. The Chicago Manual of Style Online. https://www.chicagomanualofstyle.org/tools_citationguide/citation-guide-2.html
- University of Westminster. (n.d.). Harvard referencing: Cite Them Right 13th edition. University of Westminster Library. https://libguides.westminster.ac.uk/referencing/harvard