A number of Mexican constitutional scholars have insisted, since the early 20th century, in the ephemeral nature of the original constituent power. However, they also maintain that once the original constituent power is exhausted, a permanent constituent power emerges. According to them, in the Constitution of 1917, that permanent constituent power is located in a two-thirds majority of the Federal Congress and the majority of the State Legislatures. This idea has had major legal implications. It has resulted, for example, in the judicial rejection of the doctrine of unconstitutional constitutional amendments and in a lack of differentiation between ordinary amendments and changes that alter in important ways the content of the material constitution. In this paper, we will examine the development of the notion of the permanent constituent power in Mexican constitutional theory, as well as the impact that it has had in the country’s constitutional jurisprudence.