Last updated: January 2026 · 2 min read · Audience: Authors, students, early-career researchers · Reading level: Introductory
Quick answer. A DOI (Digital Object Identifier) is a permanent, unique alphanumeric string assigned to a piece of academic content — typically a journal article — so it can be located reliably even if the publisher's website changes. The format is 10.xxxx/xxxxxxxx, for example 10.1038/nature12373. Every DOI can be turned into a clickable URL by adding https://doi.org/ in front.
This guide explains what a DOI is, why journal articles need one, how to find a DOI, how to cite a DOI in APA, MLA and Harvard styles, and how DOIs are assigned via CrossRef.
What DOI stands for
DOI stands for Digital Object Identifier. It is part of an international standard (ISO 26324) maintained by the DOI Foundation. The system was launched in 2000 to solve a problem that had become common in the early Internet era: when publishers reorganised their websites, the URLs to their articles broke, leaving every citation to those articles dead. DOIs decouple the article's identity from the URL where it currently lives. The publisher can change URLs as often as they want — the DOI remains stable.
Anatomy of a DOI
A DOI has two parts separated by a forward slash:
- Prefix (before the slash) — always starts with
10.followed by a 4–9 digit registrant code identifying the publisher.10.1038is Nature Publishing Group;10.1126is the American Association for the Advancement of Science. - Suffix (after the slash) — assigned by the publisher to identify the specific article. Suffixes can be journal-volume-page-based (
nature12373) or arbitrary identifiers (s41586-019-1644-y).
The full DOI is case-insensitive but typically displayed in lower case. Anything that looks like 10.xxxx/... is a DOI.
Do all journal articles have a DOI?
Most articles in established peer-reviewed journals published since 2000 have a DOI. Articles without a DOI are typically: very old (pre-2000), in non-indexed regional journals that have not joined CrossRef, in conference proceedings that were not formally archived, or in predatory journals that bypass the CrossRef registration process to save the registration fee. Lack of a DOI is one of the warning signs of a predatory journal.
How to find a DOI
The DOI is usually printed in one of three places:
- On the article PDF — typically on the first page near the title, in the header, or in the publication metadata block.
- On the journal's article landing page — under the abstract or in the citation block.
- In the database record — Google Scholar, Crossref Search, PubMed, Scopus, and Web of Science all display DOIs.
If you have a citation but no DOI, search Crossref directly at search.crossref.org using the article title — Crossref's database covers more than 140 million scholarly works.
Is a DOI the same as a URL?
No. A DOI is an identifier; a URL is an address. The DOI is permanent; the URL might redirect to the publisher's current page even if their domain changes. To turn a DOI into a clickable URL, prefix it with https://doi.org/ — for example, DOI 10.1038/nature12373 becomes the URL https://doi.org/10.1038/nature12373. This URL will always resolve to the article, even if the publisher changes domains.
How to cite a DOI
APA 7
Include the DOI as the final element of the reference, formatted as a clickable URL:
Patel, R., & Chen, L. (2024). Affordable open access publishing. Journal of Scholarly Publishing, 18(4), 112–128. https://doi.org/10.1234/jsp.2024.018
Do not write "doi:" or "DOI:" before the link in APA 7 — that style was deprecated.
MLA 9
Cite the DOI when the article was accessed online. Format as doi:10.xxxx/xxxxxxx or as a clickable URL:
Patel, Rajesh, and Lina Chen. "Affordable Open Access Publishing." Journal of Scholarly Publishing, vol. 18, no. 4, 2024, pp. 112–128, doi:10.1234/jsp.2024.018.
Harvard
Append the DOI as a URL at the end of the reference:
Patel, R. and Chen, L. (2024) 'Affordable open access publishing', Journal of Scholarly Publishing, 18(4), pp. 112–128. doi: 10.1234/jsp.2024.018
How DOIs are assigned: CrossRef and DataCite
DOIs are not issued by a single central authority. They are issued by Registration Agencies, the largest of which are CrossRef (for journal articles, books, and book chapters) and DataCite (for research datasets). Publishers pay a small fee per DOI to register the work and submit metadata — title, authors, abstract, references — to the registration agency. This metadata feeds Google Scholar, Scopus, Crossref Search, and most of the indexing infrastructure that makes academic research discoverable.
EP Journals Group is a CrossRef member. Every accepted manuscript is assigned a CrossRef DOI within 24 hours of acceptance, at no additional cost to the author.
Why your paper needs a DOI
A DOI gives your work three things that a plain URL cannot:
- Permanence — the DOI keeps resolving even if the journal moves, rebrands, or changes its URL structure.
- Discoverability — articles with DOIs are picked up by Google Scholar, Scopus, Web of Science, and other indexing systems via Crossref's metadata feed.
- Citation tracking — Crossref's "Cited-by" service tracks how many other papers cite yours, using the DOI as the link. Without a DOI, citation counts are incomplete.
Common DOI mistakes
- Treating DOI as optional. If a DOI exists, include it — even for print sources. APA, MLA, and Harvard all require it.
- Including "doi:" prefix in APA 7. Removed in the 7th edition. Just use the URL form:
https://doi.org/10.xxxx/.... - Treating the publisher URL as a substitute. Publisher URLs break. DOIs do not. Always prefer the DOI form.
- Confusing DOI with PMID or arXiv ID. These are different identifier systems. Articles often have all three; the DOI is the most universal.
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