This thorough study of the expression of contrast in the world's vowel systems examines phonetic & phonological differences between so-called strong & weak positions, bringing the full range of data from positional neutralization systems to bear on central questions at the interface between phonetics & 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) & original studies in experimental phonetics. Devoted at once to empirical coverage & to theoretical investigation, this is the 1st 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 & weak positions for the realization of vowel contrasts are selected & examined in detail: stressed & unstressed syllables, domain final & non-final syllables, & domain initial & non-initial syllables. Neutralization patterns in each position are extracted from survey data & analyzed in light of the phonetic characteristics of each pair of positions. Both the nature of the patterns identified as well as the variety & sources of exceptions have important consequences for formal phonology, phonetics & historical linguistics as well.