This book presents a comprehensive functional view of the phonetic implementation system, a system which is emerging in current research as a distinct level of linguistic patterning. This system performs a mapping from the post-lexical phonological structure of a language to the measurable articulatory and acoustic forms produced in actual speech of speakers of that language. The approach to this system that is taken in this work is to determine its input, its output, and the relationships between the two in the vernacular speech of four related English dialects. The endpoints of this mapping are tied down by developing a theory of the phonological input to the system in terms of the surface phonological structure of English, and a theory of the phonetic output of the system, in terms of phonetic dimensions that relate both articulation and acoustics. The mapping between the two is then studied in both phonological and acoustic-phonetic aspects in the vernacular speech of four related English dialects. This allows us to consider whether the phonetic implementation system may have some linguistic (that is, language-particular) aspects, by investigating dialects which are quite different in phonetic details. A number of important aspects of the phonetic structure (in terms of measurements of first and second formant frequencies) of the different dialects are characterized and related to one another, including the average phonetic qualities of vowel nuclei, the effects of phrasal stress on vowels, and to a much lesser extent, the effects of following consonant environments. These observed patterns by which vowels in English dialects are variably produced, reduced, and coarticulated are seen as important aspects of the system of phonetic implementation.