Iwasawa theory is a branch of number theory that focuses on the study of certain invariants associated with p-adic L-functions, where p is a prime number. It was originally developed by Japanese mathematician Kenkichi Iwasawa in the 1950s and has since become an important area of research in number theory. The central objects of study in Iwasawa theory are p-adic L-functions, which are generalizations of classical L-functions that arise from algebraic number theory. These p-adic L-functions encode information about the behavior of certain Galois representations attached to number fields, and play a key role in understanding the arithmetic properties of these fields. Iwasawa theory has connections to many areas of mathematics, including algebraic number theory, arithmetic geometry, and representation theory. It has been used to prove important results in number theory, such as the Main Conjecture of Iwasawa theory, which provides a precise relationship between the p-adic L-functions and the Selmer groups of certain Galois representations. Overall, Iwasawa theory is a deep and rich area of research that continues to be actively studied by mathematicians around the world.