This paper focuses on exploring the foundational and ambivalent status of dreams in Flaubert’s work by taking a look at the dialogue the novelist instated – on this issue – with the Romantic heritage and with scientific discourse. From the Romanticism of his early years, to the ideal of an art which could “make you dream”, through the denunciation of dreams as an illusion and a threat, dreams and dreaming are at the heart of Flaubert’s aesthetics. Familiar as he was with the medical debates of the time on the links between dreams, hallucination and madness, Flaubert recreates hallucinatory or dreamlike states with clinical acuity and uses dreams as a structuring principle in his faerie projects as in La Tentation de Saint Antoine (The Temptation of Saint Anthony). If the dream can turn into a sentimental cliché, it is also a powerful creative resource : Flaubert, conscious of both the attractions and the dangers of dreams, entertains the ideal of controlled daydream or dreaming.