  Let $M$ be a symplectic manifold, equipped with a Hamiltonian action of a
torus $T$. We give an explicit formula for the rational cohomology ring of the
symplectic quotient $M//T$ in terms of the cohomology ring of $M$ and fixed
point data. Under some restrictions, our formulas apply to integral cohomology.
In certain cases these methods enable us to show that the cohomology of the
reduced space is torsion-free.
