The »Quaestio disputata« ›About the union of the incarnate Word‹ is one of Thomas Aquinas‹ shortest Questiones. It contains, in an extremely condensed manner, the whole programme of constitutional Christology: The unity of the person and the hypostasis of Christ in two natures / Christ one or two? / The unity of esse and the duality of action and will in Christ. The last three of the five articles reveal a peculiar originality within Aquinas‹ Opus. The volume investigates the proverbial onto-logical implications of the christology of the greatest thinker of Western Christianity. It provides an edition, translation, and extensive commentary with theological and historical reflections.