This thesis is an articulatory investigation into phonological variation and change in English /l/-darkening. Although syllable-based accounts of /l/-darkening state that light [l] occurs in onsets (e.g. `leap') and a dark variant in codas (e.g. `peel'), numerous works linking phonology with other subfields of linguistics have shown that this simplified distinction cannot fully account for the variation found. Firstly, /l/-darkening is sensitive to morphosyntactic structure, as shown through overapplication of the process in certain morphosyntactically defined positions: e.g. word-finally in phrases such as `heal it', or stem-finally before a suffix in words such as `healing'. In addition, analyses of /l/-darkening from several phonetic studies have led to some arguing against an allophonic distinction altogether, stating that the difference between light and dark variants is merely two extremes of one continuum. Not only does this interpretation challenge the traditional categorisation of /l/-darkening but, given the clear sensitivity to morphosyntactic boundaries that /l/-darkening displays, it also raises questions for a modular architecture of the grammar if phonetics can be morphologically conditioned. This dissertation is an empirical analysis of /l/-darkening, presenting data from nine varieties of English. Given the difficulty in measuring liquid consonants reliably, ultrasound tongue imaging is used to provide a thorough account of the prime articulatory correlations of darkening processes. The present study provides hitherto absent instrumental evidence confirming the varying degrees of morphosyntactic sensitivity across different dialects. I demonstrate that, rather than being contradictory or chaotic, variation to morphosyntactic boundaries cross-dialectally makes complete sense under an analysis that pays due consideration to the diachronic evolution of phonological processes. Moreover, my data show that the majority of speakers display both categorical allophony of light and dark variants, and gradient phonetic effects coexisting in the same grammar. Therefore, an adequate account of English /l/-darkening presupposes both a theory of the morphosyntax-phonology interface, and the phonetics-phonology interface.I interpret these results by assuming the modular architecture of the life cycle of phonological processes, whereby a phonological rule starts its life as a phonetically driven gradient process, over time stabilising into a phonological process at the phrase level, and advancing through the grammar. Not only does the life cycle make predictions about application at different levels of the grammar, it also predicts that stabilised phonological rules do not replace the phonetic processes from which they emerged, but typically coexist with them. Moreover, the obvious intimate link between /l/-darkening and /l/-vocalisation can be explained in terms of the life cycle, in the way of lenition trajectories. The results here show that, as predicted, the more recent stage of the lenition trajectory is harsher in terms of its phonetic effect, as well as less advanced in the grammar, applying at a lower level than darkening when the two co-occur in the same variety.I conclude by arguing that the proposed analysis demonstrates that a full understanding of /l/-darkening in English requires an approach that considers variation under phonetic, phonological and morphosyntactic terms. The wide range of dialectal diversity, for which this thesis provides only a small subset, shows a great deal of orderliness when paying due consideration to the diachronic evolution of variable phonological processes.