Functionally, a DOI is a lengthy "sentence" of letters and numbers that serves to identify the unique online location of a particular journal article. This alphanumeric string of characters serves as a digital fingerprint for an article's life online, similar to a URL for a particular webpage.
Much like its title, author, and publication information, an article's DOI can be used to identify both a particular article and its set location on the internet. Although access to an article may change over time, its DOI will remain constant - so you'll always be able to find it as it lives out its digital lifespan. This is why it's important to include an article's DOI when citing it, so people reviewing your work can access your sources if they so choose.
Depending on the citation style you're using, you may be asked to include a DOI when making reference to, or citing, a specific item. If the item was published prior to 2000, it may not have a DOI attached to it (see the next tab). However, all current digitally-available items have an associated DOI that will be included in the item's catalog record, as well as on the publication itself.
As an example, this is a journal article published in the online edition of the BMJ: British Medical Journal. The publication is available to us through ProQuest, and you can see the article's catalog record below. The DOI is listed at the top of the record (red box).
The DOI will also be listed in the article's publication details, which should be available in the catalog record as well (you may need to scroll down the record to find the DOI; red box).
If the DOI is hyperlinked, this will serve as the main access point for the article's existence online. Clicking the hyperlink will automatically redirect you to a webpage where the article is available online.
Finally, the DOI is also listed on the published copy of the article (both for physical and electronic items). This is typically printed at the top or bottom of the article's first page, and will be preceded by the acronym "DOI" (red box).
Not all articles have associated DOIs, because many of them were published long before the digital age began. Although a good number of pre-digital scholarship has been made available in electronic format, this doesn't negate the fact that those items were originally published before DOIs were routinely assigned to scholarly articles (let alone created in the first place).
Sometimes, publishers, databases, and distributors will assign a retroactive DOI to an article wasn't originally published with one - meaning that the article's online "edition" will be given a DOI to demarcate its life online. But this isn't always the case. Some articles, regardless of their availability online, never receive a DOI.
But that's okay! If you come across an item that lacks a DOI, chances are it was published prior to the early 2000s, and was likely never assigned a DOI (retroactively or otherwise). Therefore, you can omit that information from your citation in good conscience.
There are two ways you can check for an article's DOI if you're not able to find it (or you're not sure it even has one).
You can visit Crossref.org, a website devoted to assisting scholars with research and reference efforts, including those related to publication and citation issues. Searching for the article by title, author, publication, etc. should provide specific publication information about the item in question. If a DOI is not included, you can confirm that there was no DOI ever assigned to the item, so you won't need to include it bibliographically.
If you're not sure about an item's DOI, try searching it in DOI.org. This website is the main hub of all things DOI, and maintains records of all items that have associated DOIs. Searching for an item by DOI should confirm that you have the correct identifier for the title in question.
