Walcher, the prior of Great Malvern in Worcestershire (d. 1135), is a landmark figure in the history of medieval science, whose work brought the Latin computistical tradition to its apex while foreshadowing the twelfth-century Renaissance in mathematical astronomy. His most famous achievement is the observation of a lunar eclipse in 1092 with the aid of an astrolabe, which is the first of its kind to be recorded in a Latin source. In spite of his renown, Walcher's writings have never received any close scrutiny and the precise rationale and modalities behind his observations and calculations remain ill-understood. This volume contains the first complete edition of Walcher's two known treatises (De lunationibus and De Dracone), together with an English translation and a detailed commentary. An introductory study will elucidate the background to his scientific pursuits and situate them in the intellectual and disciplinary context of the late-eleventh and early-twelfth century, when Latin astronomy underwent a transformative period of lasting significance.