This chapter offers the first English translation of a treatise by Dominicus Gundissalinus (ca. 1115–post-1190), On Unity and the One (De unitate et uno), completed by an introduction to the main theoretical coordinates discussed in the treatise. In his first philosophical work, Gundissalinus treats a delicate metaphysical problem: What do we mean when we say that something is “one”? Grounding his discussion on Ibn Gabirol’s Fons vitae, Gundissalinus’ answer to this question is remarkably Neoplatonic. Proceeding from the true One, unity is the efficient cause of the union of two opposite entities, matter and form, into the ontological individuality proper to each and every created thing. Gundissalinus connects this metaphysical point to various sets of problems, from the hierarchical order of the universe to the difference between discrete and continuous quantities. Accordingly, De unitate et uno is a significant example of the first Latin encounter with Jewish Neoplatonism in a fast-changing landscape that would shift to Aristotelianism only a few decades after Gundissalinus’ death.