Ernst Zermelo (1871–1953) is best-known for his great, pioneering work in set theory that transformed the subject. Of focal importance are his introduction of the axiom of choice in 1904 and his axiomatization of set theory in 1908. The axiom of choice led to a methodological enrichment of mathematics, and the axiomatization was the starting point of post-Cantorian set theory. Zermelo also made significant contributions later, particularly in his 1930 introduction of the cumulative hierarchy.