In Aquinas’ encounter with Pseudo-Dionysius can be discovered an integral philosophy of reality - a comprehensive vision of existence, depicting the universe in its procession from and return to the Absolute, according to each grade of reality, including man, its place in the hierarchy of being. The point of divergence is the primacy attributed, in turn, by the authors to the Good or to Being as a universal principle. Against this background the present work investigates the influence of Dionysius with respect to the central themes of Aquinas’ metaphysics: knowledge of the Absolute, and its nature as transcendent; Being as primary and universal perfection; the diffusion of creation; the hierarchy of creatures and return of all to God as the final end. This is one of the few studies to date which considers in a comprehensive way the relation between these remarkable thinkers. By concrete example and continual reference it illustrates both the pervasive influence of Pseudo-Dionysius and the profound originality of Aquinas.