The aim of this paper is to draw to the attention of readers not conversant with Welsh the significance and importance of a work little known at present outside Wales but deserving of a far wider readership. This is the anonymous medieval Welsh prose treatise on mysticism called Ymborth yr Enaid (Sustenance of the Soul), the third and sole surviving book of a larger work called Cysegrlan Fuchedd (Holy Living). Ymborth yr Enaid has several claims to distinction. First, it is the only example of a medieval Welsh treatise on mysticism, and for that reason, as well as its intrinsic merits, it is important in the context of medieval vernacular mystical writings. Second, it is the work of a man who was not only a considerable theologian and philosopher but also a very gifted artist. Third, the work represents a unique fusion of native Welsh bardic learning and mainstream scholastic theology, a fact which is not only remarkable in itself but also, in this case, far-reaching in its implications for the Welsh prose literature of the period generally, both religious and secular, not least with regard to such fundamental questions as the dating, provenance, and authorship of much of it. Ymborth yr Enaid is a work rich in variety and pregnant with significance.