This article is an analysis of some of the most important instances of apparitions linked with the nightmare experience in English literature during the late Middle Ages. After a general introduction on the two notions of "incubus" and "nightmare" and a brief survey of the medieval interpretations of these phenomena, I have traced some of the different variations on the beliefs associated to incubi as demonic apparitions, concentrating on the place that medieval auctoritates gave to incubi in their descriptions and classifications of the forces of evil. The article then continues with an investigation of nightmares as the most neglected subdivision in medieval theories of dreams, and with an attempt to explore the dividing line between incubi and nightmares, and the area they seem to share in medieval thought. Through these different approaches I have attempted to shed some light on this twilight zone, and establish some connections between the two phenomena. In the second part of the article, our attention is focussed on the occurrence of incubi in Middle-English literature. The only study on the subject with any claim to thoroughness is, as far as I am aware, Nicolas Kiessling's The Incubus in English Literature (1977), which attempted the herculean task of describing such occurrences in English literature from Beowulf to Byron and after. The result, in spite of a few illuminating passages, tends to be more a farrago than a catalogue, and has very little claim to critical depth. There was therefore room for a study such as the one I have undertook here. I have concentrated on three aspects: the handling of incubi or incubus-like apparitions on the part of Chaucer and some roughly contemporary texts; the apparitions narrated by Gower as the 'Tale of Nectanabus' in his Confessio Amantis; and the tradition surrounding the birth of Merlin and the belief that he was the son of an incubus. Through the analysis of these particular moments of middle-English literature I hope to offer a comprehensive view of how the late Middle Ages in England considered 'fearefull and troublesome dreames, incubus and such inconveniences' (Robert Burton, The Anatomy of Melancholy). In the Appendix I have added the transcription of two hitherto unpublished religious tales concerning incubi apparitions, from a thirteenth-century manuscript now in the British Library.