A Digital Object Identifier (DOI) is a permanent code assigned to a published article, dataset or chapter. Because publishers maintain the registry, DOIs do not break the way ordinary URLs do.

Where to find a DOI

Look near the title or copyright information on the publisher's page. DOIs usually start with 10. and look like 10.1037/0033-2909.131.6.803.

How to format a DOI

Modern citation styles (APA 7, MLA 9, Chicago 17) prefer the resolvable URL form: https://doi.org/10.1037/0033-2909.131.6.803.

If a source has both a DOI and a URL, always cite the DOI.