This chapter discusses the key role of nightmares in fantastique fiction with consideration to how nightmares were described and illustrated in other nineteenth-century sources. The genre of French fiction known as the fantastique refers to stories set in an ordinary world disrupted by an ambiguously supernatural incident that leaves the protagonist (who is usually also the narrator) scarred. A method used by many fantastic writers to introduce and maintain this ambiguity is introducing the supernatural element when the narrator is in bed and possibly asleep; the reader is therefore uncertain about whether the character is suffering through a nightmare or truly interacting with something supernatural. The chapter initially draws a contrast between nonfictional accounts of the nightmare in the nineteenth century which focus on the suffering dreamer’s unstable behaviour, and artworks depicting the nightmare which focus on illustrating the nightmare itself; fantastique texts are then identified to simultaneously depict both aspects through its ambiguity. Three characteristics are essential to nonfiction, artistic, and fantastique portrayals of the nightmare: negative emotions, vividness, and uncertainty. The chapter then investigates the application of these three characteristics to the fantastique nightmare in Théophile Gautier’s “La Morte Amoureuse” (1836) and Guy de Maupassant’s “Le Horla” (1887).