  We discuss possible signatures of quantum gravity for the propagation of
light, including an energy-dependent velocity (refractive index), dispersion in
velocity at a given energy, and birefringence. We also compare the
sensitivities of different astrophysical observations, including BATSE data on
GRB 920229, BeppoSAX data on GRB 980425, the possible HEGRA observation of GRB
920925c, and Whipple observations of the active galaxy Mrk 421. Finally, we
discuss the prospective sensitivities of AMS and GLAST.
