The book investigates historical patterns of vowel diphthongization, assimilation and dissimilation induced by consonants – mostly (alveolo)palatals – in Romance. Compiling data from dialectal descriptions, old documentary sources and experimental phonetic studies, it explains why certain vowels undergo raising assimilation before (alveolo)palatal consonants more than others. It also suggests that in French, Francoprovençal, Occitan, Rhaetoromance and dialects from northern Italy, mid low vowel diphthongization before (alveolo)palatal consonants started out with the formation of non-canonical falling diphthongs through off-glide insertion, from which rising diphthongs could emerge at a later date (e.g., Upper Engadinian OCTO ‘eight’ > [ɔc] > [ɔ(ə̯)c] > [wac]). Both diphthongal types, rather than canonical falling diphthongs with a palatal off-glide, could also give rise to high vowels (dialectal French [li] < LECTU, [fuj] < FOLIA). This same Gallo-Romance diphthongization process operated in Catalan ([ʎit], [ˈfuʎə]). In Spanish, on the other hand, mid low vowels followed by highly constrained (alveolo)palatals became too close to undergo the diphthongization process ([ˈletʃo], [ˈoxa]).