  The determination of alpha_s from hadronic tau decays is impeded by the fact
that two choices for the renormalisation group resummation, namely fixed-order
(FOPT) and contour-improved perturbation theory (CIPT), yield systematically
differing results. On the basis of a model for higher-order terms in the
perturbative series, which incorporates well-known structure from renormalons,
it is found that FOPT smoothly approaches the Borel sum for the tau hadronic
width, while CIPT is unable to account for the resummed series. An example for
the behaviour of QCD spectral function moments, displaying a similar behaviour,
is presented as well.
