The plurality of Gospel traditions constituted one of the perplexing phenomena of early Christian history. Little wonder that many attempted to explain the differences away. Gospel Harmonies were meant to demonstrate the factual unity of early Jesus traditions. Tatian has to his name one of the earliest harmonies, probably the most successful example: the so-called Diatessaron. Of this work, only indirect traces remain. This fact has made the reconstruction of Tatian's harmony one of the classic challenges for students of early Christian traditions. The present volume is a collection of studies about the quest for a lost book, a book that has been missing for over 1500 years now. It hasn't vanished, however, without a trace. We do not only know for sure that a Christian convert, called Tatian, wrote a Gospel Harmony in Rome around 172, ample indirect evidence of its existence is still available: a library of Gospel Harmonies, commentaries and translations full of probable or possible vestiges of its structure and contents. We know the title of the book - Diatessaron - and why it was written - to compose one homogeneous Gospel Harmony in the early centuries of Christianity, not only because the idea of one Gospel was attractive to many, but also because it was a quite intelligent composition. One can easily see why the Diatessaron could become a model for other attempts to construct a Gospel Harmony in later days.