In the interpretive literature from the 1950's through the 1970's the term 'criterion' was thought to be a central key to the understanding of Wittgenstein's later philosophy. Later on, it was relegated from this place of honour to being one of a variety of expressions used by Wittgenstein in dealing with philosophical questions. This Element tries to account for the shifting fate of this concept. It discusses the various occurrences of the word "e;criteria"e; in the Philosophical Investigations, argues that the post-Wittgensteinian debate about criteria was put on the wrong track by a problematic passage in Wittgenstein's early Blue Book, and finally gives an overview of the main contributions to this debate, trying to achieve a reconciliation between the rival conceptions.