Set theory was invented by Georg Cantor in a remarkable series of six seminal papers. In this chapter, we will learn notation, terminology, and basic operations of set theory. We will follow the axiomatic method by Zermelo with the amendments suggested by Fraenkel and Skolem, so that we can avoid the consistency problems of “naive set theory.” The axioms of this set theory are commonly known as ZFC, where the acronym honors Zermelo with Z and Fraenkel with F, and the letter C indicates the inclusion of the axiom of choice. This axiom schema forms the basis of most areas of modern mathematics.