Historians use two main types of sources in their research, primary sources and secondary sources. Primary sources consist of documents and other records produced during the period being studied. They include books, diaries, letters, and government records. Motion pictures and tape recordings also serve as primary sources. Secondary sources are materials prepared later by people who studied the primary sources.
Historians choose documents that reveal most accurately the facts they wish to know.
Therefore, they prefer primary sources to secondary ones, and confidential reports to public ones. Historians who study recent events use a special type of source. They go to participants in those events and record their oral testimony. Such oral history supplements documentary history. The scarcity of sources is a great problem for historians, whose work sometimes resembles that of detectives.