In joint work with Buch and Kresch, we obtained Pieri and Giambelli formulas valid in the cohomology ring of isotropic Grassmannians X = Sp_2n/P, where P is any maximal parabolic subgroup of Sp_2n. A combinatorial outgrowth of this work is a theory of theta polynomials, whose algebra agrees with the Schubert calculus on X. In the Lagrangian case, the theta polynomials coincide with the Schur Q-polynomials. We will discuss this theory and give new tableau formulas for these objects and related type C Stanley symmetric functions. We introduce the notion of a skew element w of the hyperoctahedral group, and identify the set of reduced words for w with the set of standard k-tableau on a skew shape.