The author of this volume attempts to establish a link between the notions of lenition and fortition on the one hand, and the implicational hierarchy of obstruents on the other, through the property of sonority. Earlier theories of lenition and fortition are critically assessed and the typological patterning of obstruent systems is given thorough treatment. Crucial links between these two fields of phonological phenomena are discovered, empirically verified and phonologically explained. The hypothesis is tested against a corpus of diachronic phonological changes from a large number of languages and is further demonstrated through the detailed historical discussion of the obstruent systems of the Germanic languages. In the last chapter the author proposes a model for the representation of manner and place of phonological segments which explains the idiosyncratic behaviour of palatal obstruents and correctly predicts a range of phenomena that originally fall outside the intended scope of the investigation.