At the turn of the nineteenth century, as inquiries about degeneration shaped medical, sociological and anthropological discourses, masks flourished as portraits, ornaments, and disguises. This comparative study explores tales that revolve around masks and mask-making in relation to nineteenth-century thought, offering innovative readings of fictional and dramatic works by Max Beerbohm, Gabriele D'Annunzio, Jean Lorrain, Hugo von Hofmannsthal, Andrey Bely, next to artefacts such as the plaster cast of the Inconnue de la Seine, the waxes of criminals held in Cesare Lombroso's museum, Rodin's 'horror masks' modelled after a Japanese dancer. In the fin-de-siecle imagination, the author argues, masks addressed two concepts: the repressed elements of the psyche and the perceived parameters of a declining phase in Western civilization. By uncovering the role of masks as key tropes in fin-de-siecle culture, this monograph also demonstrates to what extent the medical, anthropological and aesthetic spheres overlapped, offering insights that contribute to debates about gender and ethnicity in decadence and modernist studies.