The hadronic contribution to $(g - 2)\mu$

Abstract : The evaluation of the hadronic contribution to the muon magnetic anomaly a_mu is reviewed, including a new estimate using precise results on the pi+pi- spectral function from the KLOE Collaboration. It is found that the KLOE data confirm to some extent the previous e+e- annihilation data in this channel, and accentuate the disagreement with the isospin-breaking-corrected spectral function from tau- -> pi-pi0 nu decays. Correcting for the empirical difference in the mass of the charged and the neutral rho locally improves, but does not resolve this discrepancy. A preliminary reevaluation (including the KLOE data) of the e+e- -based Standard Model prediction of a_mu results in a deviation of 2.7 standard deviations from the BNL measurement.