De Coelesti Hierarchia (Greek, Ancient (to 1453);: Περὶ τῆς Οὐρανίας Ἱεραρχίας|Peri tēs Ouranias Hierarchias, "On the Celestial Hierarchy") is a Pseudo-Dionysian work on angelology, written in Greek and dated to ca. AD the 5th century; it exerted great influence on scholasticism and treats at great length the hierarchies of angels.
Thomas Aquinas (Summa Theologica, I.108) follows the Hierarchia (6.7) in dividing the angels into three hierarchies each of which contains three orders, based on their proximity to God, corresponding to the nine orders of angels recognized by Pope Gregory I.