Ernst Witt (1911-1991) was one of the most ingenious mathematicians of his century and has decisively shaped the development of various mathematical fields like algebra, number theory, group theory, combinatorics and Lie theory. Among his most important results are the Witt ring of quadratic forms and the ring of Witt vectors. This book includes all of Ernst Witt's papers, in addition some of his unpublished papers, facsimiles and remarks on further mathematical progress. Additional English comments by other authors provide an excellent survey on the further development of these mathematical fields. One of Witt's students, Günter Harder, contributed an essay on Witt vectors.