  Model independent constraints on the mass of extra neutral gauge bosons and
their couplings to charged leptons are given for LEP~II and a 500\,GeV $e^+e^-$
collider. Analytical exclusion limits are derived in the Born approximation.
The $Z'$ limits obtained with radiative corrections are always worse than those
calculated at the Born level. Polarized beams are only useful for degrees of
polarization essentially larger than 50\%. Known discovery limits on extra $Z$
bosons predicted by popular $Z'$ models are reproduced as special cases. The
$Z'$ constraints are compared to those predicted by four fermion contact
interactions.
