The Ecclesiastical History is the most important and extensive work of Nikephoros Kallistu Xanthopulos, who lived from about 1274 until after 1326 and is also the author of numerous other, mostly religious writings. Xanthopulos worked on the project from no later than 1309 until the mid-1320s, before he abandoned it for unknown reasons. Of at least 25 planned books, only the first 18 are completed and preserved, describing the period from Jesus Christ to about the year 600. The Ecclesiastical History is the only surviving work of its kind from the Middle and Late Byzantine periods. As the dedicatory preface to Emperor Andronikos II Palaiologos (1282-1328) and some remarks in the text show, it is written from a strictly Orthodox standpoint, rejecting any understanding with the Western Church. The work draws on the church historians of the early period, such as Eusebios, Socrates, Sozomenos, and Evagrios, quoting the older texts contained therein and adding new material mainly from chronicles and hagiographic writings. The Ecclesiastical History has survived, almost in its entirety, only in one codex of the Austrian National Library (Vindob. hist. gr. 8). The work exerted a great influence on the reception of early church history in Western Europe through its Latin translation by Johannes Langus, published in 1553. The first edition of the Greek text appeared only in 1630 in Paris and was later reprinted many times together with the Latin translation. The first volume of the new edition, which is presented here, covers the first six books of the work and the period up to the year 284.