On the Parts of Animals is at the heart and soul of Aristotle's scientific investigation of animals. It not only contains the results of his investigation of why different kinds of animals have the parts that they do; it also opens with a book devoted to laying the philosophical stones of the entire biological enterprise. Those philosophical foundations, in turn, reflect and build on Aristotle's theory of knowledge, as found in the Analytics, and his metaphysics and natural philosophy, as found in the Metaphysics, Physics, and De Anima. Whether one is interested in Aristotle the philosopher, or Aristotle the biologist, the De Partibus Animalium has a great deal to offer. The translation of the entire four books, with commentary, gives the reader an opportunity to judge the integrity of Aristotle's zoological practice in books II-IV, in light of the philosophical recommendations for such a study presented in book I. The translation aims to reflect the fine details of Aristotle's reasoning. The commentary gives line-by-line clarification of individual passages while at the same time providing an overall interpretation of Aristotle's purposes and methods.