On the Mysteries of the Egyptians, Chaldeans, and Assyrians

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 (The Egyptian Mysteries), is a work of Neoplatonic philosophy primarily concerned with ritual and theurgy and attributed to Iamblichus.

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.