  We examine here discontinuities in the metric, the antisymmetric tensor and
the dilaton field which are allowed by conformal invariance. We find that the
surfaces of discontinuity must necessarily be null surfaces and shock and
impulsive waves are both allowed. We employ our results for the case of
colliding plane gravitational waves and we discuss the SL(2,R)XSU(2)/RXR WZW
model in the present prespective. In particular, the singularities encountered
in this model may be viewed as the result of the mutual focusing of the
colliding waves.
