Video produced by the University of California, San Diego's Social Sciences & Humanities Library.

Google Scholar can be used to find primary sources. Click on the arrow to the right of the search box to open the Advanced Scholar Search window.

Primary Sources at Yale defines and explains the importance of primary sources along with a series of questions for evaluating documents.

The National History Day Research Roadmap provides a good discussion about the definition and use of primary sources.

"Primary sources provide first-hand testimony or direct evidence concerning a topic under investigation. They are created by witnesses or recorders who experienced the events or conditions being documented. Often these sources are created at the time when the events or conditions are occurring, but primary sources can also include autobiographies, memoirs, and oral histories recorded later. Primary sources are characterized by their content, regardless of whether they are available in original format, in microfilm/microfiche, in digital format, or in published format." -- Yale University

Examples of primary sources include

Five Criteria for Evaluating Web Pages from Cornell University offers an excellent guide for evaluating primary sources. 

The following sites offer guides to citing primary sources in the Chicago style:

Library of Congress

University of Wisconsin-Milwaukee

Citation at a Glance: Primary Source from a Web Site (Diana Hacker)