The Periegesis or Description of the Known World by Dionysius of Alexandria (second century ad) is the only Greek didactic poem composed to teach the Roman student elementary notions about the inhabited world. Eustathios of Thessalonike, who prior to his nomination for the position of archbishop of Thessalonike in 1175/8 was maistor ton rhetoron, the leading professor of rhetoric in Constantinople, wrote a systematic commentary on the poem in the form of parekbolai, as he had previously done for the Homeric epics. The prefatory letter addressed to John Doukas, the son of Andronikos Kamateros, suggests that the influential Kamateros family was the primary addressee of a work that certainly was also used by Eustathios in his teaching. Eustathios did not seek to correct errors and ambiguities or to question the validity of Dionysius’ vision of the world, even if he did not shrink from pointing out some of the contradictions found throughout the poem. Rather, his aim was to expand the brief information that the poem gives about towns and places, explain the poet’s stylistic decisions and specify the origin and spelling of toponyms and demonyms.