Primary sources are first-hand evidence related to the time or event you are investigating. This includes accounts by participants or observers and a wide range of written, physical, audio or visual materials created at the time or later by someone with direct experience.

In the sciences and social sciences, primary sources or 'primary research' are original research experiments, studies, or observations written about by the researchers themselves.

If a primary source is direct first-hand evidence, then a secondary source is second-hand commentary including anything that investigates, comments on, brings together, or reviews those primary sources and other secondary sources.