On the Mysteries of the Egyptians, Chaldeans, and Assyrians (Greek: Περὶ τῶν Αἰγυπτίων μυστηρίων), also known as the Theurgia and under its abbreviated Latin title De Mysteriis Aegyptiorum (On the Egyptian Mysteries; or often simply De Mysteriis), is a work of Neoplatonic philosophy primarily concerned with ritual and theurgy and attributed to Iamblichus.

This article includes a list of general references, but it lacks sufficient corresponding inline citations. (August 2023)

Porphyry is known to have had a disagreement with Iamblichus over the practice of theurgy, and the Mysteries consists mainly of Iamblichus' responses to the criticisms of his teacher.

Proclus, writing 100 years after Iamblichus, seems to have ascribed to him the authorship of the Mysteries. However, the differences between this book and Iamblichus' other works in style and in some points of doctrine have led some to question whether Iamblichus was the actual author. Still, the treatise certainly originated from his school, and in its systematic attempt to give a speculative justification of the polytheistic cult practices of the day, it marks a turning-point in the history of thought where Iamblichus stood.

There are 10 books in the work, of which the longest are Books 1 and 3. The contents of the books are.^[1]

• Book I: The soul and the gods
• Book II: Epiphanies
• Book III: Mantic ritual
• Book IV: Justice
• Book V: The nature of sacrifice
• Book VI: The process and effects of sacrifice
• Book VII: Egyptian symbolism
• Book VIII: Egyptian theology
• Book IX: The personal daemon (spirit)
• Book X: Conclusion

• Chaldean Oracles
• Enneads