Zermelo's Theorem, formulated by mathematician Ernst Zermelo, is a fundamental result in set theory. It states that every set can be well-ordered, meaning that its elements can be arranged in a sequence where every subset has a least element. This theorem is significant because it establishes a foundation for comparing the sizes of infinite sets. The proof of Zermelo's Theorem relies on the Axiom of Choice, which asserts that for any collection of non-empty sets, it is possible to select one element from each set. This connection highlights the importance of the Axiom of Choice in modern mathematics and its implications for understanding infinite sets.