  A future linear collider such as TESLA may be able to run on the Z0 resonance
with very high luminosity and polarised electron and positron beams. The
possibilities of measuring electroweak quantities with high precision are
investigated. Huge improvements with respect to the present precision can be
expected, especially for the asymmetries A_LR and A_b where beam polarisation
can be exploited. The very large sample of Z to bbbar events also allows
studies of various CP-violating b decays. The precision achievable on the CKM
unitarity triangle angles is comparable to experiments at b factories and
future hadron colliders.
