In this thesis we use Young's raising operators to define and study polynomials which represent the Schubert classes in the equivariant cohomology ring of Grassmannians. For the type A and maximal isotropic Grassmannians, we show that our expressions coincide with the factorial Schur S, P, and Q functions. We define factorial theta polynomials, and conjecture that these represent the Schubert classes in the equivariant cohomology of non-maximal symplectic Grassmannians. We prove that the factorial theta polynomials satisfy the equivariant Chevalley formula, and that they agree with the type C double Schubert polynomials of [IMN] in some cases.