  A key issue in making precise predictions in perturbative QCD is the
uncertainty in setting the renormalization scale. If in principle, the entire
perturbative series is void of this issue, in practice the perturbative
corrections are known up to a certain order of accuracy and scale invariance is
only approximated leading to the so-called scheme and scale ambiguities.
According to the conventional scale setting (CSS) this problem cannot be
avoided and the renormalization scale is set to the typical scale of a process
$Q$, and errors are estimated by varying the scale over a range of two
$[Q/2;2Q]$. This method is not void of ambiguities and leads to predictions
affected by large theoretical errors. Other strategies for the optimization of
the truncated expansion, such as the Principle of Minimal Sensitivity (PMS) and
the Fastest Apparent Convergence (FAC) criterion, have the same difficulties of
CSS and lead to incorrect and unphysical results. In general a scale-setting
procedure is considered reliable if it preserves important self-consistency
requirements such as all the renormalization group properties. Other
requirements are suggested by known tested theories, by the convergence
behavior of the series or by phenomenological results and scheme independence.
All theoretical requirements for a reliable scale-setting procedure can be
satisfied at once, leading to accurate results by using the Principle of
Maximum Conformality (PMC). This method generalizes the
Brodsky-Lepage-Mackenzie (BLM) method to all orders and to all observables and
in the perspective of a theory unifying all interactions, such as the grand
unified theory (GUT), the PMC offers the possibility to apply the same method
in all sectors of a theory, starting from first principles, eliminating the
renormalon growth, the scheme and scale ambiguities, and satisfying the QED
Gell-Mann-Low scheme in the $N_c\to 0$ limit.
