The Oneirocriticon of Achmet is a book on dream interpretation, written in Greek in the tenth century, It is the longest and most important Byzantine work on dream interpretation and has greatly influenced subsequent dreambooks in Byzantine Greek, medieval Latin and modern European languages. A comparison of the Oneirocriticon with medieval Arabic dreambooks shows that the Greek text is an adaptation of Muslim Arabic material, produced for the needs of the Christian readers of Greek. Scholars have noticed that a number of interpretations from the Oneirocriticon could also be found in the second-century ad Greek dreambook by Artemidoros, the oldest and most extensive dreambook surviving In Greek; it was until recently assumed that the Byzantine author had used the ancient Greek text in compiling his own work. However, the comparison of the Oneirocriticon with Arabic dream interpretation shows that any similarity between Artemidoros and the Oneirocriticon is not due to the direct use of the ancient Greek text by the Byzantine author. Artemidoros was translated into Arabic in the ninth century and greatly influenced Islamic dream interpretation, while the interpretations and theories on dream interpretation shared by Artemidoros and the Byzantine text should be attributed to the influence of Artemidoros on the Arabic sources on which the Oneirocriticon was based.