  This is a short review of present and future limits on new neutral gauge
bosons, in particular on hadrophilic or leptophobic $Z'$s recently proposed to
interpret the observed fluctuations of $\Gamma _{c,b}$ at LEP. Light gauge
bosons coupled to lepton number differences or to baryon number are also
examples of the model dependence of these bounds. The mixing between the $U(1)$
factors plays an important role in the phenomenology of these extended
electroweak models. Future improvements based on the analysis of precise
electroweak data are emphasized.
