1. To acknowledge your indebtedness to the words and/or ideas of someone else.
2.  To allow your reader to find what you found in the course of your research.

Plagiarism:  citing sources can help students avoid plagiarism.  Plagiarism has been defined as " using others' ideas and words without clearly acknowledging the source of that information." (Indiana University)  

To avoid plagiarism, you must give credit whenever you use

• another person's idea, opinion, or theory;
• any facts, statistics, graphs, drawings--any pieces of information--that are not common knowledge;
• quotations of another person's actual spoken or written words; or
• paraphrase of another person's spoken or written words.

Citing Machine (Landmark Project) allows the user to fill in blanks for the citation and get the proper citation for APA and MLA citations.

Citing Sources (Duke Univ.) has APA, MLA, Chicago, Turabian examples.

Sources  (Dartmouth College) has APA, MLA, Science examples.

APA Electronic References  Information excerpted from the 5th edition of the Publication Manual (© 2001).  "Because electronic media change rapidly, we will update this page regularly as there are additions, changes, or clarifications to APA style.

APA User Guide is a printable handout giving examples of citing print and electronic sources in the APA style.

MLA User Guide is a printable handout giving examples of citing print and electronic sources in the APA style.

CBE Citation Guide is a link to a style page from Ohio State University.