The intersection cohomology IH^*(X;Q) of a Schubert variety X is used in the geometric study of representations of Lie algebras and Hecke algebras. The decomposition theorem produces a powerful combinatorial machine to calculate these groups. More recently, Soergel and Mirkovic-Vilonen have shown connections between the IH with coefficients in Z or Z/pZ and questions in modular representation theory. Unfortunately the decomposition theorem fails with Z or Z/pZ coefficients. We show how the Bott-Samelson resolutions of Schubert varieties, together with localization of equivariant cohomology, gives a combinatorial framework to study these questions in the absence of the decomposition theorem.