Dreams were an important but epistemically ambiguous feature of ancient medicine and a key site of religious experience. Galen of Pergamum, whose father’s dreams were decisive in his becoming a physician, incorporates dreams into diagnostic inquiry, therapeutic innovation and theological speculation. Scholars have long treated Galen’s positive descriptions of his and others’ dreams as indications of religious commitments at odds with his avowed rationalist epistemology. This article re-examines Galenic texts on dream diagnosis, references to dream-based therapies and descriptions of his father’s dreams. Having traced Galen’s sources and descriptions of dreams, it shows that the oneiric is perfectly comprehensible within his rationalist, physiological framework. The article shifts questions of dreams’ significance from their origins to descriptions of their quality and the context of their mention. The article concludes by showing that, consonant with his own epistemological and rhetorical commitments, dreams offer the Pergamene physician confirmatory techniques, means of surprise and innovation and a rhetorical strategy for validating his knowledge, skill and standing.