The present volume contains papers of the participants in the International Press Conference on Motives, polylogarithms and non-abelian Hodge theory which took place at UC Irvine in June 1998. The conference commemorated The twentieth anniversary of the remarkable Irvine lectures of Spencer Bloch on "Higher regulators, algebraic K-theory and zeta functions of elliptic curves". The conference presented some of the best recent research in algebraic K-theory, Hodge theory, motivic cohomology and polylogarithms. The research program of the conference was organized around three main lecture series: VladimirVoevodsky taught a minicourse overviewing the recent developments in motivic cohomology and motivic homotopy theory; Don Zagier lectured on new results describing the periods of holomorphic and non-holomorphic modular forms; and Carlos Simpson lectured on the theory of geometric n-stacks and its applications to the variational aspects of non-abelian Hodge theory.