Phonologists have often held that phonetic ‘substance’ is more or less unrelated to phonological ‘form’. This view rests on assumptions about the phonetic domain that are highly questionable on empirical grounds. Evidence is reviewed suggesting that very few phonetic properties of vowels fail to serve the linguistic function of preserving and enhancing distinctiveness. Accordingly, much of what has been considered to be purely phonetic is also phonological in character; that is to say, the domains of phonetics and phonology overlap significantly. Finally, several well-known criticisms of the program of phonetic explanation in phonology are discussed and r