Since antiquity, philosophers have investigated how change works. If a thing moves from one state to another, when exactly does it start to be in its new state, and when does it cease to be in its former one? In the late Middle Ages, the "problem of the instant of change" was subject to considerable debate and gave rise to sophisticated theories; it became popular and controversial again in the second half of the twentieth century. The studies collected here constitute the first attempt at tackling the different aspects of an issue that, until now have been the object of seminal but isolated forays. They do so in through a historical perspective, offering both the medieval and the contemporary viewpoints.