These lectures draw on evidence from manuscripts in the Schoenberg Collection of the University of Pennsylvania Libraries, and they are illustrated by examples from Arabic, Greek and Latin manuscripts and early printed books. The subject matter ranges from mathematics, physics and metaphysics, to medicine, astrology and magic. Attention is paid especially to the words of the translators, glossators and commentators themselves, as they endeavour to work out the most appropriate and clearest way of conveying the subject matter of their texts to their respective audiences. Lecture 1, Monday, November 11, 2019: Fides Interpres. The different styles and approaches of the medieval translators from Arabic and Greek into Latin; from the literal to the paraphrastic; from the complete work to the epitome; revision and correction; the significance of the audience. View this video on youtube. Lecture 2, Tuesday, November 12, 2019: Sensus huius litterae est. The varieties in glossing technique; scholia versus readers’ glosses; images as glosses; conversations in the margin; continuities from Greek and Arabic into Latin; the university setting. View this video on youtube. Lecture 3, Thursday, November 14, 2019: Intentio mea in hoc libro. Long, medium and short commentaries; verse commentaries; commentaries on single books versus summae of several books; the ‘ego’ of the commentator; the scientific textbook. View this video on youtube.