Ernst Zermelo was a German mathematician born on December 27, 1871, and he is best known for his work in set theory. He introduced the Zermelo-Fraenkel set theory, which provides a foundation for modern mathematics by formalizing the concept of sets and their relationships. His work helped resolve paradoxes in set theory, such as Russell's paradox. In addition to set theory, Zermelo made significant contributions to game theory and axiomatic systems. He developed the Zermelo's theorem, which states that in any finite game of perfect information, one player can always win or force a draw. Zermelo's ideas have had a lasting impact on mathematics and logic.