What Is a DOI in a Reference: Parts, Format and How It Resolves
What the DOI at the end of a reference is, what its prefix and suffix mean, how doi.org turns it into a link, and how to write it correctly in APA Style.
What is a DOI in a reference? It is the Digital Object Identifier: a permanent ID for the work you cited, made of a prefix and a suffix separated by a slash. In APA Style it goes at the end of the entry as a link, such as https://doi.org/10.1037/ppm0000185. Opening that link takes the reader to the work, even if its web address changes.
This guide explains what each part of a DOI means, what happens when someone clicks it, and how to write it in your reference list. If a work you cited seems to have no DOI, see our guides on how to find the DOI for an old paper and how to cite a paper with no DOI.
What a DOI is
A DOI is a persistent identifier. The DOI Foundation's explanation of what a DOI is calls it "a digital identifier of an object, any object", and says DOIs "identify objects persistently."
In a reference list, that object might be a journal article, a book, a chapter or a data set. APA Style's page on DOIs and URLs describes a DOI as "a unique alphanumeric string that identifies content", which works as a persistent link to where the content is located.
The key difference from an ordinary web address is what can change. The DOI Foundation says the metadata, applications and services linked to a DOI "can be updated without having to change the DOI itself", so the identifier stays fixed. That is why a publisher can move an article to a new site without breaking the DOI in your reference.
The two parts of a DOI
Every DOI has a prefix and a suffix, separated by a forward slash. The DOI Handbook (PDF) sets out the rules for each part.
| Part | Example | What it is |
|---|---|---|
| Prefix | 10.1000 | "10" (the directory indicator), a full stop, then a registrant code |
| Slash | / | Separates prefix from suffix |
| Suffix | 182 | A string chosen by the registrant |
The example 10.1000/182 is the DOI Foundation's own, and it identifies the DOI Handbook itself. In APA's sample reference, 10.1037/ppm0000185, the prefix is 10.1037 and the suffix is ppm0000185.
The handbook says the prefix identifies the registrant: whoever requested and received the registration of that DOI. It also warns that "no definitive information can be inferred about a referent from a DOI name alone", and that a registrant code is not evidence of who owns or manages the work now. So do not use a prefix to judge a source.
Are DOIs case sensitive?
For matching, no. The handbook treats the capital and small letters A to Z as the same when testing whether two DOI names are equal.
In your reference list, still copy it exactly. APA says: "Do not change the capitalization or punctuation of the DOI or URL."
How a DOI resolves into a link
When a reader opens https://doi.org/ followed by a DOI, the DOI system looks up that string and sends them to the work. In the DOI Foundation's words, it will "look up the unique string and redirect you or your machine to the object that it points to."
The machinery behind this is the Handle System. The DOI Handbook says the DOI system "is an implementation of the Handle System", and that the https://doi.org proxy acts as a gateway between ordinary web links and that system. It adds that "Most users access the DOI infrastructure" through the web proxies at https://doi.org.
Some DOIs point to more than one location. The DOI Foundation says that in those cases, users are sent to the appropriate one.
Who registers DOIs
DOIs are issued through registration agencies. The DOI Foundation's list of registration agencies includes Crossref, which it describes as open scholarly infrastructure for research objects, and DataCite, which "provides DOIs for research data and all other research outputs."
This matters when you check a DOI. A data set's DOI may have been registered with DataCite rather than Crossref, so look up the record with the agency that registered it.
What is a DOI in a reference list in APA Style
In an APA reference, the DOI is the last element, written as a full link. The DOIs and URLs page (Sections 9.34 to 9.36 of the seventh edition manual) gives the rules, and APA's journal article examples show them in use.
Grady, J. S., Her, M., Moreno, G., Perez, C., & Yelinek, J. (2019). Emotions in storybooks: A comparison of storybooks that represent ethnic and racial groups in the United States. Psychology of Popular Media Culture, 8(3), 207–217. https://doi.org/10.1037/ppm0000185
That is APA's own example reference. The rules behind the DOI at its end are these.
- Include a DOI for every work that has one, "regardless of whether you used the online version or the print version."
- Write it in the form https://doi.org/ followed by the DOI.
- If a work has both a DOI and a URL, include only the DOI.
- You do not need "Retrieved from" before it, and you should not add a full stop after it.
- Copy and paste it from your browser "to avoid transcription errors."
- Do not break the link with a manual line break; a break added automatically by your word processor is fine.
Crossref asks for the same form. Its DOI display guidelines say DOIs should be shown as a full link starting https://doi.org/, not preceded by "doi:" and without "dx" in the domain.
How to fix DOIs in your reference list
Copied references can carry DOIs in out-of-date forms. APA says to "standardize DOIs into the current preferred format for all entries." The table uses the DOI from APA's own conversion example.
| What you have | Problem | Write it as |
|---|---|---|
| doi:10.1037/a0040251 | Old "doi:" label | https://doi.org/10.1037/a0040251 |
| http://dx.doi.org/10.1037/a0040251 | Old domain | https://doi.org/10.1037/a0040251 |
| https://doi.org/10.1037/a0040251. | Full stop after the link | https://doi.org/10.1037/a0040251 |
| Retrieved from https://doi.org/10.1037/a0040251 | Unneeded words | https://doi.org/10.1037/a0040251 |
| A DOI and a publisher URL | Two links | The DOI only |
After fixing the format, open a sample of the links. A DOI can be perfectly formatted and still belong to a different work, which our guide on how to check if a citation is real shows you how to catch.
What about shortDOIs?
APA allows them. Its page says to use the DOI Foundation's shortDOI service to create one, and that a work "can have only one DOI and only one shortDOI."
What a DOI does not tell you
A DOI tells you a work was registered and where to find it. It does not tell you the work was peer reviewed, that it is still current, or that it supports your sentence.
That is why a resolving DOI is the start of a check, not the end. Compare the page it opens with your reference, then look for any correction or retraction notice on that page.
What Thesisroom does with this
Thesisroom's Citation Check looks each reference up in eight registries: Crossref, OpenAlex, Semantic Scholar, PubMed, Retraction Watch, DOAJ, CORE and Unpaywall. It shows which references it could verify, which are retracted, and which need checking by hand.
Formatting stays with you. Use the table above to bring every DOI into one form, and APA's page for any case it does not cover.
Frequently asked questions
What does DOI stand for?
Digital Object Identifier. The DOI Foundation describes it as a digital identifier of an object that identifies it persistently, so the link keeps working even if the work moves to a new web address.
Where does the DOI go in an APA reference?
At the end of the entry, written as a link in the form https://doi.org/ followed by the DOI. Do not add a full stop after it. APA says words such as "Retrieved from" are not needed.
Do I include the DOI if I read the print version?
Yes. APA says to include a DOI for all works that have one, whether you used the online or the print version.
Can I write doi: instead of https://doi.org/?
Not in APA's current format. APA says older forms such as "doi:" and "DOI:" should be changed to https://doi.org/ followed by the DOI, for all entries. Crossref's display guidelines ask for the same.
Does every journal article have a DOI?
Not necessarily. APA's journal article page says to include a DOI "If a journal article has a DOI", so check first. Search for it before you give up, then follow APA's rules for works without a DOI if none exists.
Does a DOI mean a paper is peer reviewed?
No. A DOI identifies a registered work and links to it. The DOI Handbook says nothing definitive about a work can be inferred from the DOI name alone. Check peer review and status on the journal's own pages.
Sources
- DOI Foundation: What is a DOI
- DOI Foundation: DOI Handbook (2025, PDF)
- DOI Foundation: DOI Handbook page
- DOI Foundation: Existing registration agencies
- APA Style: DOIs and URLs
- APA Style: Journal article references
- Crossref: DOI display guidelines
How this guide was made: Thesisroom drafted this guide with the help of AI writing tools, then checked every fact, figure and source by hand against the references linked above.