  QED, Hadronic, and Electroweak Standard Model contributions to the muon
anomalous magnetic moment, a_mu = (g_mu-2)/2, are reviewed. Theoretical
uncertainties in the prediction a_mu^SM = 116591597(67) times 10^{-11} are
scrutinized. Effects due to ``New Physics'' are described. Implications of the
current experiment vs. theory constraint a_mu^exp - a_mu^SM = 453(465) times
10^{-11} and anticipated near term error reduction to +- 155 times 10^{-11} are
discussed.
