  I summarize work done with Bamert, Burgess, Nardi and London in which we
examine the effects of new physics on R_b. Our work facilitates the
identification of such new physics in case the present two-sigma deviation of
R_b from its Standard Model value should persist, or alternatively it helps to
constrain new physics in case the discrepancy continues to disappear. We
consider two general classes of effects: mixing of the b or t quarks with
heavy, exotic quarks, and loop corrections to the $Zb\bar b$ vertex. Our aim
was to give an analysis which is as exhaustive and as model-independent as
possible.
