  We analyze the divergent behaviour of the perturbative expansion in QCD,
focusing on some well-known sources of divergence such as renormalons,
instantons and the Landau pole divergence. We determine at which perturbative
order each of these sources of divergence becomes dominant. In particular, we
consider the cross section of Higgs and top production in gluon fusion at the
LHC and we construct a model for the theoretical uncertainty on the missing
higher orders in the perturbative expansion. We compare our model with some
other existing models for theoretical uncertainty.
