It is often assumed that there is a sharp division between phonetic and phonological processes, but the two are often strikingly similar, as in the case of phonetic consonant–vowel coarticulation and phonological assimilation between consonants and vowels. Parallels of this kind are best accounted for if both types of phenomena are analysed within a unified framework, so similarities result from the fact that both phonetic and phonological processes are subject to the same constraints. A unified model of phonetics and phonology is developed and exemplified through the analysis of parallel phonetic and phonological assimilation processes. The model operates in terms of scalar phonetic representations to accommodate phonetic detail, but categorical phenomena can still be derived from the interaction of speech production constraints with constraints that motivate the formation of distinct categories of sounds for the purposes of linguistic contrast.