This book develops an Optimality-theoretic model of the interaction of phonological tone with segmental sonority, arguing that tone can interact directly with vowel quality without mediating factors such as syllable structure or duration. The proposal is tested against rich and complex patterns of vowel reduction in East Slavic dialects. Though the idea that tone constitutes a part of the phonological system of some Slavic languages has been around for decades, the relationship between tone and vowel reduction has not been systematically explored in previous studies. A tone-based model developed in this book unifies many apparently disparate phenomena by proposing a limited set of constraints, whose minimal re-rankings yield the attested East Slavic vocalic patterns. On the descriptive side, this study formulates novel generalisations and presents linguistic data not previously discussed in generative linguistics. This book will be of use to students and scholars interested in phonology, Slavic languages, and the theory of grammar in general.