  I review recent estimates of the non-perturbative hadronic vacuum
polarization contributions. Since these at present can only be evaluated in
terms of experimental data of limited precision, the related uncertainties pose
a serious limitation in our ability to make precise predictions. Besides e+e- -
annihilation data also tau decay spectra can help to get better predictions.
Here, it is important to account for all possible iso-spin violations in tau -
decay spectra, from which e+e- cross sections may be obtained by an iso-spin
rotation. The observed 10% discrepancy in the region above the rho may be
understood as a so far unaccounted iso-spin breaking effect.
