  In some string theories, e.g. SO(32) heterotic string theory on Calabi-Yau
manifolds, a massless field with a tree level potential can acquire a tachyonic
mass at the one loop level, forcing us to quantize the theory around a new
background that is not a solution to the classical equations of motion and
hence is not described by a conformally invariant world-sheet theory. We
describe a systematic procedure for carrying out string perturbation theory
around such backgrounds.
