Byrhtferth of Ramsey was one of the outstanding scholars produced by the late Anglo-Saxon church; his principal work, the Enchiridion, completed in the year 1011, is a handbook designed to explain the complexities of medieval date-reckoning - called computus. The Enchiridion includes digressions on metrics, rhetoric, astronomy, and arithmology. Never before adequately edited, this new edition of a neglected late Old English scientific text throws new light on our knowledge of eleventh-century scientific scholarship. The text is accompanied by a full Introduction, apparatus criticus and facing modern English translation, detailed Commentary, and an appendix containing the Latin computus which the Enchiridion was designed to elucidate, together with glossaries of the Old English and difficult Latin words occurring in the Enchiridion itself.