  We discuss string theory relations between gravity and gauge theory tree
amplitudes. Together with $D$-dimensional unitarity, these relations can be
used to perturbatively quantize gravity theories, i.e. they contain the
necessary information for calculating complete gravity $S$-matrices to any loop
orders. This leads to a practical method for computing non-trivial gravity
$S$-matrix elements by relating them to much simpler gauge theory ones. We also
describe arguments that N=8 D=4 supergravity is less divergent in the
ultraviolet than previously thought.
