  The setting of the renormalization scale ($\mu_r$) in the perturbative QCD
(pQCD) is one of the crucial problems for achieving precise fixed-order pQCD
predictions. The conventional prescription is to take its value as the typical
momentum transfer $Q$ in a given process, and theoretical uncertainties are
then evaluated by varying it over an arbitrary range. The conventional
scale-setting procedure introduces arbitrary scheme-and-scale ambiguities in
fixed-order pQCD predictions. The principle of maximum conformality (PMC)
provides a systematic way to eliminate the renormalization scheme-and-scale
ambiguities. The PMC method has rigorous theoretical foundations; it satisfies
the renormalization group invariance (RGI) and all of the self-consistency
conditions derived from the renormalization group. The PMC has now been
successfully applied to many physical processes. In this paper, we summarize
recent PMC applications, including event shape observables and heavy quark pair
production near the threshold region in $e^+e^-$ annihilation and top-quark
decay at hadronic colliders. In addition, estimating the contributions related
to the uncalculated higher-order terms is also summarized. These results show
that the major theoretical uncertainties caused by different choices of $\mu_r$
are eliminated, and the improved pQCD predictions are thus obtained,
demonstrating the generality and applicability of the PMC.
