The issue of optimum facet reflectivity for large bandwidth semiconductor lasers is addressed. The laser facet reflectivity is chosen for maximizing the relaxation oscillation frequency, by optimization of the photon lifetime, and maximum obtainable intracavity photon density. The optimum mirror reflectivity also depends on the relative importance of photon density-induced facet damage, parasitic leakage, heating, and gain saturation. In practice, it is found both theoretically and experimentally, that higher mirror reflectivities can result in higher modulation bandwidths. The issue of optimum mirror reflectivity is also of special interest for surface emitting lasers where the cavity length is very short but the mirror reflectivity is very high.