What is the DOI?
A DOI is a unique string of letters and numbers assigned by a publisher to an article. It provides a persisent or stable link to the item on the internet. The American Psychological Association recommends using the DOI when citing documents in APA style.
Where can I find it?
The DOI is typically located on the first page of the electronic journal article, near the copyright notice. In databases such as PsycINFO, you can find the DOI on the article's detail page.
A word of caution! Online tools are excellent but they are not necessarily correct. They read data as it is passed to them, so if the data is incorrect, the citation will be too. You need to understand APA format so you can edit and proof the results of the online products.
In addition to tools like those listed below, look for the Cite feature in the databases.