This paper argues for a theory of mutation and feature-changing morphology based on floating autosegments. Three constraints governing the docking of autosegments are proposed. Phenomena discussed include inflectional suffixes in Nuer that change [voice] and [continuant], [voice] polarity in DhoLuo, the 'mixed mutation' of Breton, and vowelless roots which behave as dominant in the Chukchee vowel harmony system. An autosegmental approach is argued to provide a better account of these facts than either REALIZE-MORPHEME or anti-faithfulness.