This chapter provides an overview of the system of initial consonant mutations in contemporary Welsh, a grammatical feature which sees some word-initial sounds alternate systematically in certain lexical or syntactic contexts. The chapter includes a detailed overview of the phonological processes of mutation and of the different types of triggers of mutation. It then proceeds to a review of literature on sociolinguistic variation in mutation, commenting in particular on the factors which govern the production of nonstandard mutation as well as on diachronic changes in mutation. Observations are provided on the acquisition of the mutation system by children. The chapter finishes with suggestions for future directions for research, including methodological considerations.