Part I: The Gospel that was part of Marcion's collection in the first half of the 2nd century is the oldest of all Gospels. The author presents a meticulous reconstruction and argues that all canonical Gospels are dependent on this Gospel. This sheds new light on the formation and tradition history of the gospels, contributes new insights on the history of the New Testament text and textual criticism, and has significant consequences for how to assess the formation of the canonical edition of the New Testament and the history of earliest Christianity in general. Part I contains the methodological foundation, substantiating the hypothesis that canonical Luke is a revised edition of this oldest Gospel. On this ground, a model of the Gospels' tradition history is developed. A detailed epilogue comments on recent research on and reconstructions of Marcion's Gospel and analyzes the methodological differences of these approaches. Part II: Reconstructing the text of Marcion's Gospel is essential for the two basic tasks of establishing this Gospel's priority over canonical Luke and understanding the formation process from this oldest Gospel to the canonical Four-Gospel book. Part. II offers a reconstruction of the oldest Gospel. Its text is established on the basis of evidence provided by the heresiological witnesses and the textual variants. A detailed commentary makes every single decision of the reconstruction transparent and carefully traces the steps of the tradition history for individual sayings and pericopes. The volume includes an English translation as well as an extensive list of the correspondences between the attestations for Marcion's Gospel and the variants of canonical Luke.