This book provides the reader with a comprehensive edition of the Church Histories by John Diakrinomenos and Theodore Lector. It contains the original texts along with translations into English and a commentary. We offer a completely new edition, which should be a convenient tool to be used by the researchers. We have made a clear distinction between what is certainly the content that comes from Theodore and the later transmission of the factual information derived from his Church History as can be found in various Byzantine works. We have also attempted, in each particular instance, to trace the connection between the transmission in the later works and Theodore's composition. It should be emphasized that it is the first translation of all Theodorean tradition into modern language.