October 26, Leiden, Room 411 Hendrik Lenstra 11:00-12:00: Construction of the Witt ring 16:00-17:00: Witt rings and zeta functions These lectures are devoted to the `universal' Witt ring and one of its applications. In the first lecture, which is of a tutorial nature, the Witt ring is constructed in a manner that is relatively free of formulas, and the Adams operations are introduced. The second lecture makes a connection with Grothendieck rings of group representations and gives an application to zeta functions.