This article attempts an overview of the function of dreaming throughout the Byzantine period. With the onset of Christianity dreams remained a form of communication between the natural and the supernatural worlds. The new religion incorporated their ancient functions and aetiologies reshaping them according to its own principles. Two divergent attitudes can be detected in religious literature: Ascetic sources promote a restrained, negative dream theory which emphasises the demonic character of dreaming, while hagiography treats most dreams as god-sent, accepting their prophetic nature. During the iconoclastic controversy dreams join forces with icons in their struggle to gain status and acceptance. With the end of iconoclasm the ancient tradition of oneiromancy resurfaces, enriched by Islamic borrowings, through the circulation of a number of dream-books (oneirocritica). The function of oneiromancy in the Middle and Late Byzantine periods is compared with that of astrology. It focusses on the act of the oneirocritical consultation (i.e. the narration and professional interpretation of dreams), a process which resembles modern psychoanalysis, attending to the personal, everyday needs and aspirations of the individual dreamer.