It is well-known that the early assignment of prominence in sequences like THIRteen MEN vs. thirTEEN, (defined as the Rhythm Rule, or post-lexical stress shift), is an optional phenomenon. This dissertation examines some of the factors that encourage the application of stress shift in English and how it is phonetically realised. The aim is to answer two sets of questions related to why and how stress shift occurs in English: 1a) Does prosodic boundary strength influence stress shift? 1b) Does the adjacency of prominences above the level of the segmental string encourage stress shift? 2) How is stress shift realized? a) Is stress shift only a perceptual phenomenon? and b) Which syllables, if any, change acoustically when stress shift is perceived? To answer these questions, four experiments were designed. The first three experiments test whether the strength of the prosodic boundaries before and after the target word (e.g., canteen) influence stress shift. The effect of the strength of the left-edge prosodic boundary was investigated by comparing perceived stress patterns of the target (e.g., canteen) as produced in isolation where it is preceded by an utterance- and a phrase- initial prosodic boundary (the Isolated condition) with its rendition when embedded in a frame sentence (e.g., Say canteen again) where the left prosodic boundary before canteen is weaker (the Embedded condition). Results show a very clear tendency towards late phrasal prominence on the final accentable syllable (e.g., -teen in canteen) in the Embedded condition while in the Isolated condition this pattern appeared in less than half of the targets, showing that the stronger left boundary increased the incidence of stress shift. Two more experiments manipulated the strength of the boundary to the right of the target (#) respectively by changing the syntactic parse of the critical phrase (e.g. canteen cook) in sequences like (1) and by manipulating constituent length as in (2). Results showed that the syntactic manipulation significantly affected the strength of the prosodic boundary between the clashing words which was stronger in (1b) relative to (1a), and affected the incidence of stress shift, which was higher in (1a) relative to (1b). The length manipulation also affected the rate of stress shift, which was significantly higher in the phrase with the shorter word, e.g., soups (2a) relative to phrase with the longer word, e.g., supervisors (2b). (1) Example from the Syntax Experiment a. Who is the canteen (#) cook these days? (Pre-modifier + Noun) b. How do the canteen (#) cook these days? (NP + VP) (2) Example from the Length Experiment a. It should include the canteen (#) soups again. (Shorter constituent) b. It should include the canteen (#) supervisors again. (Longer constituent) Whilst we knew from the literature that the grouping of the clashing words within one Intonational Phrase (IP) encourages stress shift, results from the Syntax and Length experiments indicate that this (i.e., the phrasing of the clashing words within same IP) is not sufficient condition for the occurrence of stress shift, and that fine-grained degrees of boundary strength below the Intonational Phrase can drive changes in prominence pattern. The fact that higher rates of stress shift (and associated significant acoustic changes) were driven by manipulations of constituent length --for sequences with the same syntactic structure-- provides support for the idea that prosodic (rather than syntactic) boundaries directly influence stress shift. The fourth experiment tests the definition of stress clash in English in cases like fourteen candles where the two main lexical prominences are strictly adjacent along the time dimension, in fourteen canoes where the prominences are not adjacent in time, but adjacent at the higher levels of the metrical hierarchy, and in fourteen canteens where the main lexical prominences are not adjacent, and do not clash. This experiment highlighted and resolved an unacknowledged disagreement about what clash status sequences with one weak intervening syllable (e.g., fourTEEN caNOES). The fourTEEN caNOES type were shown to behave like metrically clashing sequences (e.g., fourteen CANdles) in attracting stress shift, and differently from the non-metrically-clashing sequences (e.g., fourteen CANTEENS) in discouraging it. These results provide empirical support for the Standard Metrical Theory (e.g. Selkirk, 1984; Nespor & Vogel, 1989) claim that 1) stress clash matters in triggering stress shift and that 2) stress clash in English is defined at the higher prosodic levels and not restricted to the level of the segmental string as indirectly assumed in a growing body of research (e.g., Vogel, Bunnel & Hoskins, 1995; Tomlinson, Liu & Fox Tree, 2014). Along with the establishment of prosodic boundary strength as one of the predictors influencing stress shift, another important contribution of the thesis is providing empirical evidence that the English Rhythm Rule is not solely a perceptual phenomenon and that it is associated with acoustic correlates. The main correlates of perceived stress shift consistently appearing across experiments is the decrease in the duration of the main lexical prominence of the target (e.g., -teen in canteen) and the increase of fundamental frequency and Sound Pressure Level peaks and on the initial syllable (e.g., canin canteen), when followed by a main clashing phrasal prominence. The acoustic analysis shows that the first accentable syllable also contributes in the perception of stress shift. This latter result does not lend support to the deletion formulation of the Rhythm Rule (Gussenhoven, 1991) which stipulates that the impressions of stress shift are solely associated with changes of prominence in the last accentable syllable of the target (e.g. -teen in canteen). Along with the determination of the acoustic correlates of perceived stress shift in English, the present research 1) indicates that fine-grained gradations of prosodic boundary strength can influence stress shift, 2) shows that while stress clash can increase the incidence of stress shift, stress shift can take place even in environments completely free of stress clash, and 3) provides evidence that stress clash should not be construed simply as the concatenation of two main lexical prominences along the time dimension.