Whenever you quote or base your ideas on another person's work, you must acknowledge the source you used. Even when you do not quote directly from another work, if reading that source contributes to the ideas presented in your paper, you must give the author(s) proper credit.
Citations allow readers to:
In written academic work, citing sources is standard practice and shows that you are responding to this person, agreeing with that person, and adding something of your own. Think of documenting your sources as providing a trail for your reader to follow to see the research you performed and discover what led you to your original contribution.
Publication Manual of the American Psychological Association (APA)
We have two copies in the Folsom Lake College Library. One at the Research Help Desk & the other on the Writing Guides Stand.