  QED, Hadronic, and Electroweak Standard Model contributions to the muon
anomalous magnetic moment, a_mu = (g_mu-2)/2, and their theoretical
uncertainties are scrutinized. The status and implications of the recently
reported 2.6 sigma experiment vs.theory deviation a_mu^{exp}-a_mu^{SM} =
426(165) times 10^{-11} are discussed. Possible explanations due to
supersymmetric loop effects with m_{SUSY} \simeq 55 sqrt{tan beta} GeV,
radiative mass mechanisms at the 1--2 TeV scale and other ``New Physics''
scenarios are examined.
