  Spontaneous breaking of parity or time reversal invariance offers a solution
to the strong CP problem, the stability of which under quantum gravitational
effects provides an upper limit on the scale of symmetry breaking. Even more
important, these Planck scale effects may provide a simple and natural way out
of the resulting domain wall problem. (Invited talk at the 4th Hellenic School
on Elementary Particle Physics, Corfu, Greece, September 1992.
