Abstract. Using hypercohomology, we can extend cyclic homology from algebras to all schemes over a ring k. By 'extend' we mean that the usual cyclic homology of any commutative algebra agrees with the cyclic homology of its corresponding affine scheme.The purpose of this paper is to show that there is a cyclic homology theory HC * of schemes over a commutative ring k, extending the usual cyclic homology HC * of k-algebras. By a cyclic homology theory for schemes over k we mean a family of graded k-modules HC n (X) associated to every scheme X over k which satisfy:(0.1) they are natural and contravariant in X; (0.2) for each affine scheme X = Spec A, there are natural isomorphismsWe discuss uniqueness of a cyclic homology theory briefly in Remark 0.5 below. We have chosen homological indexing because of axiom (0.2), and because cohomological indexing (HC n = HC −n ) would concentrate the nonzero groups in negative degrees.The construction is simple: just sheafify Connes' (b, B) double chain complex and take Cartan-Eilenberg hypercohomology (as defined in the appendix). Axioms (0.1) and (0.3) are immediate from generalities about hypercohomology of unbounded complexes. The point of this paper is that axiom (0.2) also holds. In the spirit of Grothendieck's seminal letter [GdR], Geller and I also proved in [WG] that sheafifying and taking hypercohomology yields a "Hochschild homology