ÆLfsīDen occurs in three different remedies, each in a different collection, though of these two must be textually related: one of the two remedies in Lācnunga which contain ælf (section 29, ff. 137r–138r); section 41 of Leechbook III (ff. 120v–121r); and a related remedy in Book I of Bald's Leechbook (section 64, ff. 52v–53r). Unfortunately, the textual contexts of ælfsīden provide little unequivocal evidence for its meaning, while the word sīden occurs only in ælfsīden. However, sīden is almost certainly cognate with the Old Norse strong verb síða (to give a broad and advised translation, ‘work magic’), and its derivatives seiðr (the magic worked) and síði (the magic-worker): it derives from the infinitive stem of síða's Germanic ancestor *sīþanam, with the deverbative nominal suffix -en. Síða is, as a strong verb, a priori likely to have an Indo-European origin. It has phonologically and semantically convincing Indo-European cognates in Welsh hud (‘magic’), hudo (‘work magic, work by magic’) and Lithuanian saĩsti (‘intepret a sign, prophesy’). The word síða and probably its basic meaning originate, then, in a pre-Germanic ancestor found in other Western Indo-European languages. These words probably derive from an Indo-European root concerning binding. As with ælfādl, discussed in the last chapter, the determiner ælf- in ælfsīden probably denotes the source of the sīden; if so, ælfsīden probably meant something along the lines of ‘the magic of ælfe’. Sīdsa, also attested in an ælf-remedy (in Bald's Leechbook II, section 65, f. 106r), is another cognate, with the deverbative suffix -sa, and is accordingly considered here too. I begin by analysing the texts which attest to sīdsa and ælfsīden in detail, in ascending order of complexity; I then proceed to the textually related remedy Wið ælfcynne (Leechbook III, section 61, f. 123). Having re-assessed our core Old English evidence for sīdsa and ælfsīden, I then broaden the scope to draw in a comparative context: primarily Scandinavian material concerning seiðr, but also medieval Irish and Middle English material. This allows us to develop a sense of the narratives with which ælfsīden is likely to have been associated in Anglo-Saxon culture – and their possible social meanings.