The introduction illustrates the prominence and pervasiveness of nightmares in the culture, literature, and arts of the long nineteenth century. Starting from an acknowledgement of the central and uniquely fascinating place that this period holds in the Western history of dreams and, specifically, nightmares, it foregrounds the inherent plurality that characterised nineteenth-century definitions, notions, and theorisations of nightmares as not only dreams of oppression and terror but also disquieting oneiric experiences in-between sleep and wake and mysterious supernaturalSupernatural presences attacking vulnerable sleepers. It then offers an overview of some crucial themes explored by nineteenth-century representations of nightmares, including the limits of consciousnessConsciousness and reasonReason/rational, the complex interplay of bodyBody and mindMind/mental, the elusive boundaries between inner and outer worlds, and the dread of (unconsciousUnconscious, sexualSex/sexuality/sexual/erotic, racialRace/racial, supernaturalSupernatural) alterityAlterity/otherness. While the examples of this section are mostly drawn from English-speaking countries and France, reflecting the current state of investigations into the subject, the introduction concludes by showing how the nineteenth-century history of nightmares extended across these geographic and linguistic boundaries, to the whole Europe and beyond, and by arguing for the importance of a cross-disciplinary and cross-national wide-ranging approach to fully appreciate and illuminate the significance of nightmares for this period.