The book is a collection of articles which aims at providing an overall treatment of the question of Lenition/Fortition in its many aspects: historical, typological, synchronic, diachronic, empirical and theoretical. Various current approaches to phonology are represented. The first part of the book is intended to provide stable and, if possible, pre-theoretical criteria that allow to characterize phenomena which count as Lenition/Fortition, and to work out the properties of the phenomena under study. The second part of the book deals with Lenition in selected language families. The third and final part provides formal accounts of Lenition/Fortition.