This volume presents the results of the ESAP-HYELE conference on "Aristotle, Physics 7.3: What is Alteration?", which took place in Vitznau, Switzerland, 12-15 February 2007. The contributors are part of a team of Aristotelian scholars who came together for the first time in 1995, and have since been meeting every spring. The purpose of their gatherings is to read and interpret line by line a short, but important chapter of Aristotle's works. In this way, attention is focussed on key texts of particular exegetic and theoretical interest. Each session starts with the presentation of a translation and a first analysis of the main problems; these then become the subject of an intense debate which illustrates the different schools of thought and methodological approaches. Over the years, the confrontation of these different points of view has had a beneficiary effect on scholarship and has stimulated research activity worldwide. On the occasion of the Vitznau meeting in 2007, it was decided for the first time to publish the results of the meeting in order to make them accessible to a wider public of scholars and students. The present volume is the fruit of this common effort.