Precision tests of the electroweak interaction at the Z pole

The measurements of the properties of the $Z$ boson performed with the large data samples collected at LEP and SLC challenge the standard model of the electroweak interaction with unprecedented precision. The $Z$ mass is measured to 2 parts in ${10}^5,$ while other relevant electroweak observables such as the electroweak mixing angle, which is related to the strength of the neutral current, are measured with an accuracy of 1 part in ${10}^3.$ At this level of precision the effects of electroweak radiative corrections and in particular of the nontrivial loop contributions are visible. Assuming the validity of the standard model, the top mass can be predicted with a precision of about 10% and with a value in good agreement with the direct measurements. The global fit of electroweak data constrains the mass of the Higgs boson, giving an indirect indication of a relatively light Higgs. The overall agreement of the data with the predictions of the standard model is good, considerably limiting the room available for new physics. This paper describes the experimental techniques that led to such a thorough test of the electroweak theory. The basic theoretical concepts are reviewed and the measurements compared with theoretical predictions.