This is one of the crucial texts in Aristotle's theory of change, in which he deals with the question of what alteration is and what it is not. Aristotle discusses change in various parts of his writings, and seems to provide a broad range of notions: movement and change of place, alteration in aspect and form, temporal change, variation in the way a given being is perceived, the change in relationship between beings, qualitative and accidental alterations. This volume presents the results of the ESAP-HYELE conference on "Aristotle, Physics 7.3: What is Alteration?", which took place in Vitznau, Switzerland in 2007. The contributors are part of a team of Aristotelian scholars that first came together in 1995, and have since been meeting every spring. The purpose of their gatherings was to read and interpret line by line a short, but important chapter of Aristotle's works. In this way, attention was focussed on key texts of particular exegetic and theoretical interest. Each session started with the presentation of a translation and a first analysis of the main problems; these then became the subject of an intense debate which illustrates the different schools of thought and methodological approaches. This volume sets out to provide the reader with new insights into Aristotle's: Physics 7.3.