A constitutional amendment is a modification of the constitution of a polity, organization or other type of entity. Amendments are often interwoven into the relevant sections of an existing constitution, directly altering the text. Conversely, they can be appended to the constitution as supplemental additions (codicils), thus changing the frame of government without altering the existing text of the document.

Most constitutions require that amendments cannot be enacted unless they have passed a special procedure that is more stringent than that required of ordinary legislation. Examples of such special procedures include supermajorities in the legislature, or direct approval by the electorate in a referendum, or even a combination of two or more different special procedures. A referendum to amend the constitution may also be triggered in some jurisdictions by popular initiative.

Australia and Ireland provide examples of constitutions requiring that all amendments are first passed by the legislature before being submitted to the people; in the case of Ireland, a simple majority of those voting at the electorate is all that is required, whereas a more complex set of criteria must be met in Australia (a majority of voters in a majority of states is also necessary). Switzerland has procedure similar to that of Australia.

The special procedures for the amendment of some constitutions have proven to be so exacting, that of proposed amendments either few (8 Amendments out of 44 proposed in Australia), or none (as in Japan) have been passed over a period of several decades. In contrast, the constitution of the U.S. state of Alabama has been amended 928 times since its adoption in 1901.