This dissertation argues that the affrication of /t/ and /d/ before /ɹ/ is an active sound change in progress that has been phonologized. Despite numerous references to it in the literature, no experimental work has been undertaken to investigate the phenomenon. This dissertation aims to fill that gap. Conducted over three separate studies, the research presented in subsequent chapters explores three specific questions: Q1: Apparent Time Study – Is the phenomenon of /tɹ/ and /dɹ/ affrication in English a sound change in progress? Q2: Production Study – Are English /t/ and /d/ in /tɹ/ and /dɹ/ clusters articulated like prevocalic [t] and [d], like prevocalic [tʃ] and [dʒ], like neither, or like both? Q3: Perception Study – Do English speakers categorize affricated variants of /t/ found in /tɹ/ clusters as T or CH? Chapter 2 presents results from an Apparent Time Study, which examines sociolinguistic interview data from a corpus of Raleigh, North Carolina English speakers of different ages, all born in the 20th century. The Raleigh corpus is considered to be a suitable choice for conducting this investigation for several reasons. First, it is expected that the phonetic motivation for /tɹ/ and /dɹ/ affrication will be present in any English-speaking community. Second, Raleigh experienced an influx of technology workers to the area from Northern regions of the United States in the middle of the 20th century, following the development of Research Triangle Park (RTP), making it plausible that the resulting dialect contact may have led to the introduction of novel affricated variants to the region. Third, /tɹ/ affrication has been implicated in s-retraction (in /stɹ/ clusters), which is a sound change in progress that has been reported in Raleigh English. The Apparent Time Study aims to determine whether /tɹ/ and /dɹ/ affrication, if present, is a sound change in progress and/or whether the emergence of affricated variants may have been the result of dialect contact. Building on these findings, Chapter 3 presents results from a Production Study conducted in Raleigh, North Carolina, which captures audio, ultrasound, and video data in order to investigate how English speakers’ /tɹ/ and /dɹ/ sequences are coarticulated. The Production Study provides an opportunity to find out how affricated variants of /t/ and /d/ before /ɹ/ are articulated. Chapter 4 presents results from a Perception Study, which explores how listeners (from the Production Study) categorize affricated variants of /t/ spliced from before /ɹ/. Chapter 5 compares results from across studies, and Chapter 6 provides a general discussion and conclusion.