What is the relationship between phonetics and phonology? Are phonological features innate and universal, and do they have fixed phonetic correlates? These questions have recently received renewed prominence in theoretical debates, and this book explores them from a modular, substance-free perspective. This in-depth analysis of Breton serves not only to introduce previously underused data into the theoretical landscape but also to demonstrate the viability of a modular framework for phonology. The book introduces a minimalist system of phonological representations built up on a language-specific basis, without regard to the phonetic realisation of phonological objects, and integrates it with a fully-fledged computational framework and a stratal interface between phonology and morphosyntax, showcasing the numerous empirical and conceptual advantages of a substance free view of phonology.