This book explores the writing of church history during the early Byzantine period, reconsidering the evidence for the nature and authorship of a hypothetical 'Arian' source for many surviving medieval histories of the fourth century. It considers surviving ecclesiastical histories written between the fifth and early thirteenth centuries to draw out commonalities apparently owed to this 'lost' source and discusses attempts by modern historians to reconstruct it. In doing so, it convincingly argues that this 'Arian' material likely belongs not to one work, but three: two chronicles and a martyrology. This book therefore provides a vital reassessment of fourth-century Christian historiography, as well as important insights on chronicle writing in the Middle Ages.