In this Chapter for the Routledge Handbook on Comparative Constitutional Change, I explain the functions of formal amendment rules—not of formal amendments but of the rules by which codified constitutions are altered. I also critique existing classifications of formal amendment rules, and suggest the beginnings of a more comprehensive account of their structure. My principal purpose in this Chapter is to map the scholarly understanding of formal amendment rules in order to better appreciate their various forms and functions. My second purpose is to help generate an agenda for further research into the structure, purposes and uses of formal amendment. I identify patterns, similarities, and distinctions in order to encourage renewed interest in the study of formal amendment as a subject separate from though of course related to informal amendment. This Chapter is therefore both an inquiry and an invitation to further research.