Nennius (lat. Nennius) is a Welsh historian of the 8th-9th centuries, author of the “History of the Britons” (lat. Historia brittonum), which describes the history of Britain from the settlement of the island to the era of King Arthur (VI century) and provides separate data on later periods. The name and biography of the Welsh scientist Nennius are controversial: in the preface he calls himself a student of Bishop Elvodug (Elwod), whose death in 809 is reported in the Welsh annals. “The History of the Britons” came in several editions: lengthy, short with a preface and short without a preface. Nennius’s work remains one of the main written sources on the history of early medieval England. The History of the Britons tells of Roman rule, the raids of the Picts, Scots and Saxons and their struggle with the Britons, the appearance of Germanic tribes (Angles and Saxons) in Britain, and the Anglo-Saxon conquest of Britain. In fact, the History of the Britons is a compilation from Jerome, the Irish and Anglo-Saxon annals, and the epic poems of the Britons; in addition, it contains one of the earliest versions of the story of the origins of the Irish, which was later developed in the Book of Conquests of Ireland. The narration is unsystematic and unreliable. For the first time, the legend of “King Arthur” appears, whom Nennius calls by the Roman name Artorius, considering him the leader of the Britons, who won twelve victories over the Germans. The last among them is at Mount Badon. Legendary tradition connects Artorius with Ambrosius Aurelian. Nennius tells in detail the legend about Hengest and Khors. In addition, the History of the Britons sets out the genealogy of the Anglo-Saxon kings up to 796. The medieval Irish translation of Nennius (the so-called Lebor Breatnach) is also a valuable historical source.