The amendment of treaties is of infinitely more concern than any question of their possible invalidity (Chapter 17). A subject of great practical importance, amendment always needs to be thought about seriously when drafting a multilateral treaty: afterwards is just too late. Although amending a bilateral treaty is not technically difficult, amending a multilateral treaty can raise a multitude of problems, both technical and political. In domestic law, most contracts are between two parties and are relatively short term. In contrast, a multilateral treaty may have as many as 190 or so parties, and be of unlimited duration. These factors lead to three basic problems. First, the process of agreeing on amendments and then bringing them into force can be as difficult as negotiating and bringing into force the original treaty, and sometimes even more troublesome. Secondly, because of their long life, multilateral treaties are more likely to need amendment. Thirdly, because of an inadequate amendment provision (or indeed no provision) in the original treaty, most amendments do not bind all the parties.

Before that watershed, the Second World War, treaty amendment usually required unanimity. It was relatively rare for a multilateral treaty to have a built-in amendment procedure, and when it did, the procedure would normally incorporate the unanimity rule. But, because of the difficulty of obtaining unanimity, a practice gradually developed by which amendments entered into force between only those states willing to accept them.