  We study the one-loop corrections to the $Zb\bar{b}$ vertex in extensions of
the Standard Model with arbitrary numbers of scalar doublets, neutral scalar
singlets, and charged scalar singlets. Starting with a general parameterization
of theories with neutral and singly-charged scalar particles, we derive the
conditions that, in a renormalizable model, must be obeyed by the couplings in
order for the divergent contributions to cancel. Then, we show that those
conditions are indeed obeyed by the models that we are interested in, and we
write down the full finite expression for the vertex in those models. We apply
our results to some particular cases, highlighting the importance of the
diagrams with neutral scalars.
