Secondary sources are documents or recordings that relate or discuss information originally presented elsewhere. They are one step removed from primary sources, which are immediate, first-hand accounts of a topic, from people who had a direct connection with it. Secondary sources can include most books about a topic, analysis or interpretation of data, scholarly or other articles about a topic, especially by people not directly involved, and documentaries.

Secondary sources involve generalization, analysis, interpretation, or evaluation of the original information. They often attempt to describe or explain primary sources. Secondary sources are considered to be persuasive and offer an analysis, interpretation, or a restatement of primary sources. They can be tricky to determine, but they are essential to scientific and educational communities.