  A summary of the current understanding of methods of analytical higher order
perturbative computations of total cross sections and decay widths in Quantum
Chromodynamics is presented. As examples, the total cross section in electron
positron annihilation, the hadronic decay rates of the tau lepton and Higgs
boson up to O(\alpha_s^2) and O(\alpha_s^3) are considered. The evaluation of
the four-loop QED \beta - function at an intermediate step of the calculation
is briefly described. The problem of renormalization group ambiguity of
perturbative results is considered and some of the existing prescriptions are
discussed. The problem of estimation of theoretical uncertainty in perturbative
calculations is briefly discussed.
