The recipes that form the subject of this edition have been taken from four manuscripts: British Library Additional 14912 (BLAdd), Cardiff 3.242 (Hafod 16, Card), Oxford Bodleian Rawlinson B467 (Rawl), and Oxford Jesus College 111 (the Red Book of Hergest, RBH). All four manuscripts are roughly contemporary, all dating from the end of the fourteenth century or the beginning of the fifteenth.1 In the past, scholars and commentators have tended to treat the corpus of texts which appears in these four manuscripts as a single body of material, a single text, called Meddygon Myddfai (‘The Physicians of Myddfai’). This tendency has characterised manuscript catalogues and secondary literature, but it is mainly due to the way that these texts have been presented in editions.The medical texts from RBH were edited by John Williams ‘ab Ithel’ and published under the auspices of the Welsh Manuscripts Society in 1861, along with a translation by John Pughe, in a volume called The Physicians of Myddvai. The volume contains editions of two texts: the first of these is the medical compendium from RBH, and the second is a medical compendium attributed to ‘Hywel Feddyg’ based on a copy of the manuscript provided to the editor by the great literary forger Iolo Morganwg (i.e. Edward Williams, 1747–1826).2The attribution to ‘Hywel Feddyg’ in this text is based on a note at the end of the collection, where the compiler identifies himself by name and claims descent from Einion ab Rhiwallon, one of the Physicians of Myddfai. Another note claims that the text was copied by William Bona from the book of John Jones, a physician from Myddfai and the last of the line, in 1743.3 In actuality, the book of Hywel Feddyg is based on a manuscript in the hand of the eighteenth-century scribe William Bona of Llanpumsaint (NLW 13111 part ii), which Iolo Morganwg has altered in order to make it look like an older and more authentic text.4 Iolo rearranged the contents to make them look more like a planned medical compendium, replaced much of the English vocabulary with Welsh words to make the text appear older and more authentically Welsh, left out some remedies that were obviously more recent than the date he had in mind for this collection, and added numerous short texts to the end of the compendium.5 These include a plant-name glossary which contains a number of unique, idiosyncratic or perhaps merely erroneous plant identifications, a tract on weights and measures, a list of anhepcorion Meddyg (‘the things a physician should not be without’), the ascription to Hywel Feddyg, and William Bona’s claim to have copied the text from John Jones. Iolo Morganwg’s doctored version of this compendium survives today as NLW 13111 part i, making it easy to see how he has changed the text. In reality, William Bona’s collection (that is NLW 13111 part ii) is a typical early modern medical compendium containing a mixture of medieval remedies and more recent material, and as such is worthy of further study in its own right, but it is not, as it has been presented in this edition, a compendium collected by one descendant of the Physicians of Myddfai, and copied from a manuscript belonging to another such descendant. This claim is never made by William Bona, but rather is part of Iolo Morganwg’s intentional recasting of this collection.