This thorough study of the expression of contrast in the world's vowel systems examines phonetic and phonological differences between so-called strong and weak positions, bringing the full range of positional neutralization data to bear on central questions at the interface between phonetics and phonology. Evidence is drawn from diverse sources, bringing together cross-linguistic typological surveys, detailed investigations of the diachrony of specific languages and original studies in experimental phonetics. This is the first work to compile so exhaustive a study of positional neutralization, and its theoretical conclusions bear equally on phonology, phonetics, and historical linguistics.