De Coelesti Hierarchia (Celestial Hierarchy) is a Pseudo-Dionysian work on angelology (dated to ca. the 5th century) which exerted great influence on scholasticism. It treats at great length the hierarchies of angels.
The work has also been very influential in the development of Orthodox Christian theology.
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 St Gregory I.