The Letter and Prologue on Easter of Theophilus of Alexandria (385-412), the 95-year list of Paschal data compiled by Cyril (412-444), and the Prologue or Praefatio to that list written in Latin about 482 in the persona of Cyril are among the foundational documents for our knowledge of the Alexandrian Easter cycle. That cycle, through the Latin versions of Dionysius Exiguus, Bede, and others was the standard method for determining the date of Easter in the westernchurches until the end of the sixteenth century. There has been no modern critical edition of either Prologue since those of Bruno Krusch in 1880. This new edition of the texts is based on Alden A. Mosshammer's discovery or rediscovery of manuscript witnesses unknown to Krusch and overlooked by morerecent scholars who have engaged these texts.The historical introduction summarizes current knowledge about the history of Easter calculations in early Christian communities, including a new hypothesis attributing the Alexandrian cycle in its final form to the mathematician and astronomer Theon of Alexandria working in the 370's. Although both texts have already been translated into English, Mosshammer's new translations are based on his new reconstruction of the texts. The commentaries address many issues currently under debate inhistorical scholarship, such as the origin of 21 March as the conventional date of the vernal equinox. The newly reconstructed text of the Prologue attributed to Cyril and Mosshammer's extensive commentary make that difficult text intelligible for the first time.