What can we learn from the $Z\to b\bar{b}$ vertex?

I stress the fact that a complete study of possible New Physics effects in the $Z\to b\bar{b}$ vertex requires a combined analysis of the ratio $R_b=\mathrm{\Gamma }(Z\to b\bar{b})\mathrm{/}\mathrm{\Gamma }(Z\to hadrons)$ and the longitudinally polarized forward backward asymmetry, $A_b$. This is illustrated with a number of models such as the two Higgs-doublet model, the MSSM, Technicolour and models with an extra $Z^{\mathrm{\prime }}$. The use of effective lagrangean techniques is briefly discussed.