A DOI is a Digital Object Identifer. It is a unique identifying number/charactoer string for an electronic article, ebook, or other "object". It provides a permanent, stable access to these objects on the internet.
All DOI numbers begin with a 10, such as: 10.1159/000330840 or 10.1289/ehp.1003206.
Several citations styles now incorporate DOIs for electronic sources when present. Not all articles have DOIs.
If you are unable to locate the DOI for a source, try CrossRef :
• Click on the link on the left column “Simple Text Query”
• Create a free account (this involves entering an email address, designating your use: student, faculty, etc., and entering the captcha phrase. You will be sent a link via email to click to confirm)
• Go back to the entry form and enter your registered email address to authenticate
• Enter citations in the box (copy and paste works easiest)
• Click on enter
• If a doi is available, it will appear under the citation