We review the general analysis of the contributions of electroweak vacuum-polarization diagrams to precision experiments. We first review the representation of these contributions by three parameters S, T, and U and discuss the assumptions involved in this reduction. We then discuss the contributions to these parameters from various models of new physics. We show that S can be computed by a dispersion relation, and we use this technique to estimate S in technicolor models of the Higgs sector. We discuss the reliability and the gauge invariance of this estimate. Finally, we present the limits on S and T imposed by current experimental results.