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.
Check out the Tutt Library History: Primary Sources research guide for additional books, databases, resources, and information.
Five Criteria for Evaluating Web Pages from Cornell University offers an excellent guide for evaluating primary sources.
From Yale University
"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."
Video produced by the University of California, San Diego's Social Sciences & Humanities Library.
World War I Posters in Special Collecions
Examples of primary sources include: