Kenkichi Iwasawa was one of the most original and influential mathematicians of the twentieth century. He made a number of fundamental contributions in group theory and algebraic number theory. In group theory, he created the theory of (L)-groups (including the structure theorem called "Iwasawa decomposition"), which played an important role in the solution of Hilbert's Fifth Problem. In number theory, he constructed a beautiful theory on Z_p-extensions, now called "Iwasawa theory", realizing the deep analogy between number fields and algebraic function fields. Iwasawa theory has had a strong influence on the recent development of arithmetic algebraic geometry, including the solution of Fermat's Last Theorem. This volume of the collected papers of K. Iwasawa contains all 66 of his published papers, including 11 papers in Japanese, for which English abstracts by the editors are attached. In addition, the volume contains 5 papers unpublished until 2001. Also included is a masterly summary of Iwasawa theory by J. Coates (The University of Cambridge).