The Muon Anomaly: Experiment and Theory

A summary is given of the present status of the theory and experiment of the anomalous magnetic moment of the muon. A difference between predicted and measured values is an indication of physics beyond the Standard Model. A new experimental number has produced a value which differs from a recent Standard Model prediction by about 1.6 standard deviations. When the new result was first announced, the discrepancy was about 2.6 standard deviations, but theorists have recently found an error in the sign of the largest term in the Standard Model hadronic light-by-light contribution which reduces the difference. Additional data are being analyzed and elements of the theory are being scrutinized to provide, in the future, a sharper test of theory.