Below are some common situations in which you must cite a source. This is not an exhaustive list -- there are many other possible situations in which a citation would be necessary.
Remember: If you’re not sure whether something should be cited, cite it.
It never hurts to add an extra citation.
You must cite the source when you...
Paraphrase someone’s ideas.
Mention someone’s ideas.
Summarize a source.
Quote someone’s exact words. (In addition to citing the source, you must also indicate that the words are a quotation, and not your own words.)
Use numerical data, such as statistics.
Use an image, such as a picture or a diagram.
Use multimedia, such as a video, an animation, or an audio recording.
Mention a fact that is not common knowledge. (For more about “common knowledge,” see “What is common knowledge?” in the Questions & Answers section below.)
What is a source?
If someone else created it, and you’re using it in your work, it’s a source. Lots of things can be sources, including books, articles, websites, manuscripts, photographs, films, speeches, music, interviews, emails, videos, phone calls, tweets, and many more.
What is common knowledge?
Here is the definition of “common knowledge,” taken from the UMW Honor System Guidebook and Constitution:
Common knowledge -- information quickly accessible to the reader of a given document. The reader need not actually know the information, but must be able to check its accuracy quickly in any good library without citations from the author of the paper. Usually, the reader should also be able to check or pursue the information in a variety of published sources. Such common or readily available information does not require documentation. But if the reader does need to use a particular source to follow up or verify the information, a reference to that source is necessary.
It’s often difficult to determine whether something counts as common knowledge. If you’re unsure, play it safe -- cite it!
What if I reuse an idea or sentence from a paper that I wrote for a previous class? Do I need to cite my own paper?
You may not reuse content from your previously submitted work unless you have your professor’s permission. If you do have your professor’s permission, then you may reuse your own content, but you should include a citation to your previously submitted work, so that your reader knows where the content originated.
If I didn’t use a particular source, do I still need to put that source in my bibliography?
In most cases, no. Your bibliography should only include sources that you use. However, there are exceptions to this rule, especially for higher-level work such as senior theses. If you’re not sure, talk to your professor.
This page was inspired by, and draws content from, the following webpages:
Academic Integrity at Princeton: When to Cite Sources (PDF)
Yale College Writing Center: Warning: When You Must Cite
Purdue Online Writing Lab: Is It Plagiarism Yet?
For further information, see these webpages, and also the various style manuals in the library.