This article examines the Old English word mære, the etymon of nightmare, and its variants. I address a number of questions arising from our basic Old English data in order to underpin future efforts to interpret the Old English material. Four main issues are tackled. Firstly, the existence of a strong noun mær as well as the weak mære (§2). Secondly, the source and significance of the unique lemma in the gloss incuba: mære, satyrus. This was almost certainly a glossed text of Isidore’s Etymologiae in which incubi had been corrupted to incubae, a conclusion allowing us to infer with confidence precisely how the glossator understood incuba when he chose to deploy mære as a gloss (§3). Thirdly, the gendering of the beings denoted by mære, emphasising the complexity of the evidence but suggesting the probability that maran were invariably female (§4). Lastly, the meaning of the lemma and the significance of the gloss in Echo: wudumær. The long-standing interpretation of wudumær to mean ‘echo’ can be dispensed with: it implies instead the nymph Echo, a supernatural female understood to be associated with woods (and possibly seduction), and is probably a gloss-word (§5).