The present book describes the sound changes undergone by the vowels and consonants of Catalan since the origins of the language, relates them chronologically and deals with the articulatory and acoustico-perceptual factors involved in their inception. The reconstruction of the sound change pathways is based on graphemic data from Old Catalan corpora, linguistic evidence from other Romance languages, mostly Occitan, and well-established mechanisms of vowel and consonant production. Special attention is paid to some major phonetic pathways which have operated on spoken Catalan since the early Middle Ages: the rising of stressed mid low vowels before palatal consonants to high vowels presumably through the formation of rising diphthongs (pit PeCTU, ull oCULU); the inversion of stressed mid high and mid low front vowels after /e/ centralization into schwa in the Eastern dialect ([e] bé BeNE [] sec SiCCU); extensive unstressed vowel syncope/apocope followed by changes in manner of articulation and voicing in the emerging syllable-final consonants; the historical development of the voiced dental fricative (raó RATIONE, malesa MALiTIA) and of voiced palatoalveolar affricate and fricative consonants.