The term constitutional amendment usually refers to a special procedure by which a constitution may be altered.
A constitution, particularly when it is written document, is often entrenched in some way, thus preventing from being changed by the expedient legislative process used for all other rulemaking. This entrenching may take the form of special second order rules within the constitution specifying the process for changing it — for example, requiring a particular portion of the membership to be given timely notice with an opportunity to vote on the change, not merely by a vote of the governing body; or the governing body may need a qualified (e.g. two-thirds) majority in order to pass the amendment.
The technicalities of amending a constitution vary; some examples follow.
- In the United Kingdom, which has no written constitution, but subscribes to parliamentary supremacy, the legislature has the power to enact any law it wishes, even if rules that are considered to be part of the British constitutional law would be affected. On the one hand, this prevents changes to the constitution from being considered, in some way, a "sacrilegious" action; this model may however fail to adequately protect fundamental rights of individuals and minorities against the tyranny of the majority, though not all constitutional scholars agree with this position.
- With the United States Constitution, amendments are appended to the original text, which in itself is not modified. One can therefore easily track all changes, but on the other hand cannot be sure whether a particular article, or even a previous amendment, is still in effect. Amendments must be initiated either by a two-thirds vote in each house of Congress or by the legislatures of two-thirds of the states, which may thus ask Congress to call a national convention to discuss and draft amendments. In either case, amendments must have the approval of three-fourths of the states before they enter into force.
- Other countries require that constitutional amendments alter the text of the constitution itself. For example, according to Article 79, the Basic Law of Germany can only be modified by the Bundestag (parliament) and the Bundesrat both passing a bill with a two-thirds majority that explicitly lists what text is to be replaced and the replacement texts. This makes the bill itself unreadable, but keeps the actual constitution up-to-date. In addition, the amendment of certain articles is made impossible altogether.