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 data from positional neutralization systems to bear on central questions at the interface between phonetics and phonology. The author draws evidence from a diverse array of sources, bringing together cross-linguistic typological surveys, detailed investigations of the diachrony of specific languages (Slavic, Turkic, Uralic, Austronesian, among many others) and original studies in experimental phonetics. Devoted at once to empirical coverage and to theoretical investigation, this is the first work to compile so exhaustive a study of positional neutralization patterns in the languages of the world. On the basis of this catalog of evidence, the author argues for a diachronically oriented approach to the phonetic motivations behind phonological patterns, with phonologization as its central mechanism. Three pairs of traditionally-identified strong and weak positions for the realization of vowel contrasts are selected and examined in detail: stressed and unstressed syllables, domain final and non-final syllables, and domain initial and non-initial syllables. Neutralization patterns in each position are extracted from survey data, and analyzed in light of the phonetic characteristics of each pair of positions. Both the nature of the patterns identified as well as the variety and sources of exceptions have important consequences for formal phonology, phonetics, and historical linguistics as well.