Weak neutral currents in e+e- experiments

Experimental results on weak neutral currents in e+e- experiments at PETRA and PEP are reviewed. The precise measurements of the leptonic reactions e+e- → e+e-, μ+μ- and /tau/+/tau/- are used to determine the weak neutral current parameters. The main result is the observation of a significant forward-backward asymmetry in the angular distribution of e+e- → μ+μ- and e+e- → /tau/+/tau/-, which measures directly the strength of the neutral weak interaction with respect to the electromagnetic interaction. Finally, first attempts are made to extend these studies to the reaction e+e- → q-q → hadrons with the aim of measuring the weak neutral current couplings of heavy quarks