[en] Expectations for constraints on extra Z bosons are derived for LEP 2 and future linear e⁺e^- colliders. For typical GUTs, a Z' with M_Z'≤3 to 6√s may cause observable effects. The Z' discovery limits are dominated by statistical errors. However, if a Z' signal is observed, the discrimination between different models becomes much worse if systematic errors are taken into account. Discrimination between models is possible for M_Z'<3√s. A determination of Z' anti f couplings independently of models becomes attractive with future colliders. Anticipated bounds are determined. (orig.)