Topics in Two-Loop Superstring Perturbation Theory

In this contribution to the Proceedings of the Conference on Analysis, Complex Geometry, and Mathematical Physics, an expository overview of superstring perturbation theory to two loop order is presented to an audience of mathematicians and physicists. Recent results on perturbative supersymmetry breaking effects in Heterotic string theory compactified on Z_2 \times Z_2 Calabi-Yau orbifolds, and the calculation of the two-loop vacuum energy in these theories are discussed in detail, and the appearance of a new modular identity with respect to Sp(4,Z)/Z_4 is reviewed.