This thesis produces a definition of consonant mutations, a hitherto relatively unexplored phenomenon in typology, using a sample of languages from the Celtic and Uralic languages. It is defined using Canonical Typology. The base of the phenomenon is established as ‘functionalized consonant alternations’, with seven dimensions of variation: conditioning elements>no conditioning elements; sole functional indicator>accompanied by other morphemes; more than two grades>two grades; some effect on surrounding vowels>only affecting the consonant; only leniting>also non-leniting sound changes; word-initial/-final>word-medial placement; and less regular>more regular. It is also argued that mutations’ phonology is less important than their function.