This dissertation addresses the broad question about how phonology and phonetics are interrelated, specifically how phonetic language changes, which gradually alter the phonetics of speech sounds, affect the phonological system of the language, and vice versa. Some questions I address are: (i) What aspects of speakers' knowledge of their language are changing during a phonetic change? (ii) What is the relative timing of a phonetic change and phonological reanalysis? (iii) Can a modular feed-forward model of phonology and phonetics account of the observed patterns of phonetic change? (iv) What are the consequences of my results for theories of phonology, phonetics, and language acquisition? (v) What unique insight into the answers to these questions can the study of language change in progress give us over other methodologies? To address these questions, I drew data from the Philadelphia Neighborhood Corpus [PNC] (Labov and Rosenfelder, 2011), a collection of sociolinguistic interviews carried out between 1973 and 2013. Using the PNC data, I utilized a number of different statistical modeling techniques to evaluate models of phonetic change and phonologization, including standard mixed effects regression modeling in R (Bates, 2006), and hierarchical Bayesian modeling via Hamiltonian Monte Carlo in Stan (Stan Development Team, 2012). My results are challenging to the conventional wisdom that phonologization is a late-stage reanalysis of phonetic coarticulatory and perceptual effects (e.g. Ohala, 1981). Rather, it appears that phonologization occurs simultaneously with the onset of phonetic changes. I arrive at this conclusion by examining the rate of change of contextual vowel variants, and by investigating mismatches between which variants are expected to change on phonetic grounds versus phono- logical grounds. In my analysis, not only can a modular feed-forward model of phonology and phonetics account for observed patterns of phonetic change, but must be appealed to in some cases. These results revise some the facts to be explained by diachronic phonology, and I suggest the question to be pursued ought to be how phonological innovations happen when there are relatively small phonetic precursors.