The Insular Celtic languages, such as Irish and Welsh, distinctively feature a morphophonemic process known as initial consonant mutation. Essentially the initial sound of a word changes due to certain grammatical contexts. Thus the word for 'car' may appear as carr, charr and gcarr in Irish and as car, gar, char and nghar in Welsh. Originally these mutations result from assimilatory phonological processes which have become grammaticalized and can convey morphological, semantic and syntactic information. This paper looks at the primary mutations in Irish and Welsh, showing the phonological changes involved and exemplifying their basic triggers with forms from the modern languages. Then it explores various topics related to initial consonant mutations including their historical development and impact on the grammatical structure of the Celtic languages. This examination helps to clarify the existence and operations of the initial mutations and displays how small sound changes can have a profound impact upon a language over time.