1. It gives credit to the person who created the idea. Failure to cite your source could be construed as plagarism.
2. It allows your readers to locate the sources you used, therefore they can read or judge them for themselves.
How do I cite sources?
Use the citation styles page to find out how to cite in the following styles:
What style do I use?
Check with your professor to see if a particular style is recomended for your assignment.