This paper brings Calcidius' 4th. c. AD Latin commentary on Plato's Timaeus into the fold of research on the methodological assumptions and hermeneutical practices of the ancient commentary tradition. The first part deals with the question of how Calcidius sees his role as a commentator in relation to the original text, to his audience, and to the Platonist tradition. The second part examines the organizing principles and structuring devices of the commentary, and what these can tell us about connections between exegesis and worldview. As with many other commentaries, Calcidius' purpose becomes clearer if we approach him from a pedagogical angle. His practice, like most of the content of his commentary, appears to connect him to older layers of Platonism, pre-dating so-called Neoplatonism. It reveals a distinct authorial voice, of someone who is very conscious of his role as a cultural mediator and who has a philosophical line to pursue.