 One important mode of philosophical expression from the end of the Hellenistic period and into Late Antiquity was the philosophical commentary. During this time Plato and Aristotle were regarded as philosophical authorities and their works were subject to intense study. This entry offers a concise account of how the revival of interest in the philosophy of Aristotle that took place toward the end of the Hellenistic period eventually developed into a new literary production: the philosophical commentary. It also follows the commentary tradition into Late Antiquity in order to account for the vitality and richness of this tradition. Special emphasis is given to the ancient study of the Categories because this short but difficult treatise played a central role in the post-Hellenistic return to Aristotle. The reader should keep in mind that the study of Aristotle in the form of the commentary did not mean the cessation of genuine philosophical thought. Quite the contrary: philosophers customarily used the commentary format not only to expound the works of Aristotle, but also as a vehicle for original philosophical theorizing. One consequence of this approach was that, at least at the beginning, the return to Aristotle did not involve the acceptance of any definite set of doctrines. More directly (and more boldly): there was no definite set of doctrines that a Peripatetic philosopher in the 1st century BCE was expected to accept. At the time Aristotle was considered an authority not because he was above criticism, but because he deserved to be studied carefully.