Historians all agree that the Cárdenas presidency was a crucial period in the development of twentieth-century Mexico. They would not agree as to the reasons for its importance. The range of interpretations is so wide and, at times, so nuanced, that it is risky to try to summarise the underlying disagreements. However, there are certain key differences which can be emphasised; and I shall begin this article with a quick review of what I consider those key differences to be.