Abstract
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 τ decay spectra can help to get better predictions. Here, it is important to account for all possible iso-spin violations in τ-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 π may be understood as a so far unaccounted iso-spin breaking effect.
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