This book offers an important study of Aristotle's theory of the sense-organs. It aims to answer two questions central to Aristotle's psychology and biology: why does Aristotle think we have sense-organs, and why does he describe the sense-organs in the way he does? The author looks at all the Aristotelian evidence for the five senses and shows how pervasively Aristotle's accounts of the sense-organs are motivated by his interest in form and function. The book also engages with the celebrated problem of whether perception for Aristotle requires material changes in the perceiver. It argues that, surprisingly to the modern philosopher, nothing in Aristotle's description of the sense-organs requires us to believe in such changes.