Michael of Ephesus' commentary on Aristotle's On the Generation of Animals is the earliest surviving, and perhaps first, commentary on this foundational zoological treatise. Composed in the 12th century as part of the Aristotelian revival which took place under the patronage of Constantinople's Anna Comnena, this commentary represents the state of the art of Byzantine and ancient scholarship on the philosophical questions concerning the origins and development of life. Translated here for the first time into a modern language, Michael's commentary on Books I-II of Aristotle's On the Generation of Animals focuses on Aristotle's core philosophical commitments concerning animal generation: the parts of animals responsible for reproduction, the contributions of males and females, the role of nature and the divine, the creation of different kinds of soul, and the stages of embryonic development. Throughout the commentary, Michael offers unique and insightful readings of Aristotle's text and records the opinions of his predecessors and contemporaries on questions of biology. The treatise is vital reading for those studying Aristotle's biology as well as the Byzantine renaissance of biological inquiry.