Iwasawa theory is the study of arithmetic objects over infinite towers of number fields. The main example is the extension of $\mathbb{Q}$ by the $p$-power roots of unity. The theory origins in the following insight of Iwasawa: instead of working with a fixed finite Galois extension and modules under its Galois group, it is often easier to describe every Galois module in an infinite tower of fields at once. This talk will be an introduction to Iwasawa theory.