The article seeks to summarise recent research carried out by the author into thirteenth and fourteenth-century commentaries on the De longitudine et brevitate vitae. The texts of some representative commentaries are examined as a means of assessing the reception of Aristotle's natural philosophy in the thirteenth and fourteenth centuries. As this is an area which has received comparatively little attention from researchers up to now, it is hoped that in examining commentaries on this one text of the Parva naturalia what emerges might serve to give a clearer picture of the reception and understanding of Aristotle's natural philosophy. The article also contains some brief comments on the two medieval translations of the De longitudine, by James of Venice and William of Moerbeke. In an appendix, a transcription of the prologue to Walter Burley's commentary is also included. Â© 2003 Koninklijke Brill NV, Leiden.