Effects of Extra LightZBosons in Unified and Superstring Models

We discuss the low-energy effects of extra light $Z$ bosons in unified models especially in those models which might arise from the $E_8\otimes E_8^{\prime }$ superstring. We find that deviations from the standard model in neutral-current scattering data can give a very sensitive test of the presence of such bosons, of unification, and of the pattern of symmetry breaking. Such deviations have already in fact been observed. We also discuss flavor-changing effects such as $\mu \to 3e$.