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A DOI, or digital object identifier, is a unique alphanumeric string assigned to an online document such as a journal article or ebook. A DOI can be used in a similar way to a URL, but it is more permanent and reliable. Because a DOI never changes, academic citation styles recommend using DOIs instead of URLs to locate sources when possible.

DOIs can be formatted in various ways:

• doi.org/10.3390/economies7040098
• https://doi.org/10.3390/economies7040098
• https://dx.doi.org/10.3390/economies7040098

The DOI will usually be clearly visible when you open a journal article:

You can use DOIs when citing online journal articles or certain books. Follow the standard rules for citing that type of source in your citation style, and place the DOI in the space where the URL would usually go.