  The Standard Model prediction for the magnetic moment of the muon requires a
determination of the electromagnetic form factor of the pion at high precision.
It is shown that the recent progress in pi-pi scattering allows us to obtain an
accurate representation of this form factor on the basis of the data on e+e- ->
pi+ pi-. The same method also applies to the form factor of the weak vector
current, where the data on the decay tau -> pi- pi0 nu are relevant.
Unfortunately, however, the known sources of isospin breaking do not explain
the difference between the two results. The discrepancy implies that the
Standard Model prediction for the magnetic moment of the muon is currently
subject to a large uncertainty. Talk given in May 2002, in honor of Arkady
Vainshtein.
