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This thesis is an extensive acoustic study of a range of phonetic and phonological phenomena traditionally referred to as unstressed vowel reduction. Five geographically contiguous varieties of two typologically distinct and genealogically unrelated languages, Bulgarian and Turkish, are investigated: two standard varieties, West Bulgarian and Istanbul Turkish, and three varieties from northeast Bulgaria: the Bulgarian and Turkish of a bilingual community, and monolingual East Bulgarian. Vowels are modelled as three-dimensional objects defined by duration, F1 and F2 frequencies. Unstressed Vowel Shift or the systematic differences between stressed and unstressed realisations, categorical target split and contrast neutralisation are implicationally related yet distinct aspects of traditional vowel reduction which are addressed separately. It is demonstrated that Unstressed Vowel Shift may, but does not have to, result in the phonologisation of a separate unstressed target, which in turn may, but again does not have to, merge with the target of another vowel. East Bulgarian exhibits the highest degree of overall reduction: Unstressed Vowel Shift is strong and categorical, and results in the unstressed merger of all open–close vowel pairs. Istanbul Turkish lies at the opposite end of a reduction continuum, with only gradient Unstressed Vowel Shift under temporal pressure. West Bulgarian shows different degrees of reduction across vowel pairs. The bilingual varieties reveal intricate patterns of interference: Bilingual Turkish is influenced by East Bulgarian and exhibits greater reduction than both Istanbul Turkish and Bilingual Bulgarian, the latter displaying little reduction as a result of substratal Turkish transfer. In addition to providing an in-depth analysis of Unstressed Vowel Shift in multiple varieties, the thesis investigates claims of typological incompatibility between vowel harmony and reduction, Unstressed Vowel Shift in the context of language contact, the phonological status of Unstressed Vowel Shift, the nature of phonological rules, and emergent features, which prove useful for modelling processes at different stages of phonologisation, as well as hybrid structure that arises in language contact. A number of previous claims about Bulgarian and Turkish phonology are refuted.