  The phenomenological constraints on extra neutral gauge bosons at present and
at future colliders are reviewed. Special attention is paid to the influence of
radiative corrections, systematic errors, and kinematic cuts on the Z'
constraints. Simple estimates of the Z' constraints from different reactions
are derived. They make the physical origin of these constraints transparent.
The results existing in the literature are summarized and compared with the
estimates. The consequence of model assumptions on the Z' constraints is
discussed. The paper starts with an overview of Z' parameters and the possible
links between them by model assumptions. It continues with a discussion of Z'
limits and Z' measurements in different reactions at e+e- and mu+mu- colliders.
It follows an overview of the corresponding limits at proton colliders.
Possible Z' constraints from other reactions as ep collisions, atomic parity
violation, neutrino scattering and cosmology are briefly mentioned.
