  Assuming that the scale of new physics exceeds the weak scale, we have
considered all possible origins of deviations from the Standard Model in
$\gmu2$. If the underlying theory can be treated perturbatively, then to
account for an effect as large as has been recently reported\cite{BNL}, the
only possibilities would be models that contain particles that yield direct
contributions, of which SUSY models are prototypical, or models having
unexpectedly light leptoquarks or having a rather exotic heavy vector boson
that couples predominantly to muons. Other possibilities either are already
excluded by existing experimental results or would have to be unnaturally
large. If, on the contrary, the underlying new physics involves strong
interactions, as in technicolor models, then the discrepancy could be accounted
for by a variety of effective operators.
