There has been extraordinary attention devoted to the Celtic mutations over the years, with various authors arguing for phonological, morphological, or lexical treatments (and various blends thereof). Strikingly, this literature is virtually bereft of any mention of the phonological restrictions that can sometimes limit the applicability of mutation. In this article, we provide a detailed experimental and corpus-based investigation of the phonological restrictions on Scottish Gaelic mutation. Using both techniques, we show that the phonological restrictions are alive yet are in a state of flux. The continued productivity of these phonological aspects of the mutation system argues that any analysis of mutation must attend to them.