  The evaluation of the hadronic contribution to the muon magnetic anomaly
$a_\mu$ is revisited, taking advantage of new experimental data on $e^+e^-$
annihilation into hadrons: SND and CMD-2 for the $\pi^+\pi^-$ channel, and
\babar for multihadron final states. Discrepancies are observed between KLOE
and CMD-2/SND data, preventing one from averaging all the $e^+e^-$ results. The
long-standing disagreement between spectral functions obtained from $\tau$
decays and $e^+e^-$ annihilation is still present, and not accounted by
isospin-breaking corrections, for which new estimates have been presented. The
updated Standard Model value for $a_\mu$ based on $e^+e^-$ annihilation data is
now reaching a precision better than experiment, and it disagrees with the
direct measurement from BNL at the 3.3$\sigma$ level, while the $\tau$-based
estimate is in much better agreement. The $\tau$/$e^+e^-$ discrepancy, best
revealed when comparing the measured branching fraction for $\tau^- \to \pi^-
\pi^0 \nu_\tau$ to its prediction from the isospin-breaking-corrected $e^+e^-$
spectral function, remains a serious problem to be understood.
