This book sets out to explain developments in the field of non-linear phonology that have taken place since the mid 1970s. It sees phonology as compromising several tiers, each tier consisting of a linear arrangement of segments; these are linked to each other by association lines which indicate how they are to be co-articulated. Originally devised to handle tonal phenomena, the approach has been extended to deal with other features whose scope is more than one segment, especially vowel and consonant harmony. The book opens with a discussion of tonal systems, moving on to a look at problems of vowel length and geminate consonants from an autosegmental perspective and to introduce the notion of a skeletal tier. The third chapter deals with the treatment of the syllable leading to a discussion of metrical theory. The fifth chapter considers lexical phonology and the final chapter considers some of the more interesting developments in the field.