Revision happens when a text is changed. Its most common name in Greek was διόρθωσις; in Latin, emendatio. It was practised by writers of all styles and levels of ability, working alone and in consultation with others, and in many different genres. Evidence for revision comes from papyri and from descriptions in ancient literature. It occurred on papyri, in wax tablets, and in authors’ minds as they prepared a text, and it was understood by ancient writers as either the inevitable consequence of error or as a valuable exercise leading to greater cognitive and political skill. In addition to reminding us of the fluidity of textuality and the always contingent nature of every literary formation, the study of revision provokes reflection on the relationship between literature and natural language, and on writing’s place in social exchange.