This chapter discusses archival research, starting with a description of archives. The various types of archives constitute a precious resource for researchers, and in many disciplines, such as sociology, political sciences, law, or history, archival research occupies a central place. Depending on the field of research, analysis may be limited to a single archive or require a comprehensive comparative analysis of various archives, taking into account their diverse nature and purposes. Archival research can be defined as finding, using, and correlating information within primary and secondary sources. Primary sources are sources created by persons directly involved in the event, reflecting their point of view, such as personal diaries or letters. In contrast, secondary sources are sources not based on a direct observation of an event, or on evidence directly associated with the subject, and they rely on pre-existing primary sources. The chapter then details the main steps of archival research and looks at the development of online archives and databases.