In her introduction to the most recent translation of the eleven Middle Welsh prose tales, known as Mabinogion, Sioned Davies summarises their main themes by pointing out that ‘[t]hey tell of love and betrayal, shape-shifting and enchantment, conflict and retribution.’ Indeed, shapeshifting is a recurring phenomenon in these stories: in six of the eleven tales we find a total of 23 transformations of different types. These metamorphoses tell us, as Caroline Walker Bynum, Marina Warner, and Laurence Harf-Lancer have taught, about ways of thinking about change and identity, and the boundaries between human and animal realms, and between the natural and the supernatural. However, they have not yet been systematically addressed. The following pages will survey metamorphoses in secular and religious prose tales written in Wales, both in Middle Welsh and Latin, paying particular attention to vocabulary, with the aim of discussing their narrative function and meanings. In section one I will examine instances of shapeshifting found in a group of Middle Welsh tales, which are associated with magic and the marvellous. In addition, a few cases of transformations are narrated in hagiographical texts as supernatural miracles performed by or for saints; they will be discussed in section two. This comparison is especially elicited due to the interplay of Latin and vernacular writing, as well as of religious and secular texts, and the particular character of the lives of Welsh saints.