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 $Z \rightarrow b\bar{b}$ vertex requires a combined analysis of the ratio $R_b= \Gamma(Z \rightarrow b\bar{b})/ \Gamma(Z \rightarrow hadrons)$ 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 $Z'$. The use of effective lagrangean techniques is briefly discussed.