This is the first study to assess in its entirety the fourth-century Latin commentary on Plato's Timaeus by the otherwise unknown Calcidius, also addressing features of his Latin translation. The first part examines the authorial voice of the commentator and the overall purpose of the work; the second part provides an overview of the key themes; and the third part reassesses the commentary's relation to Stoicism, Aristotle, potential sources, and the Christian tradition. This commentary was one of the main channels through which the legacy of Plato and Greek philosophy was passed on to the Christian Latin West. The text, which also establishes a connection between Plato's cosmology and Genesis, thus represents a distinctive cultural encounter between the Greek and the Roman philosophical traditions, and between non-Christian and Christian currents of thought.