  A key problem in making precise perturbative QCD predictions is the
uncertainty in determining the renormalization scale $\mu$ of the running
coupling $\alpha_s(\mu^2).$ The purpose of the running coupling in any gauge
theory is to sum all terms involving the $\beta$ function; in fact, when the
renormalization scale is set properly, all non-conformal $\beta \ne 0$ terms in
a perturbative expansion arising from renormalization are summed into the
running coupling. The remaining terms in the perturbative series are then
identical to that of a conformal theory; i.e., the corresponding theory with
$\beta=0$. The resulting scale-fixed predictions using the "principle of
maximum conformality" (PMC) are independent of the choice of renormalization
scheme -- a key requirement of renormalization group invariance. The results
avoid renormalon resummation and agree with QED scale-setting in the Abelian
limit. The PMC is also the theoretical principle underlying the BLM procedure,
commensurate scale relations between observables, and the scale-setting method
used in lattice gauge theory. The number of active flavors $n_f$ in the QCD
$\beta$ function is also correctly determined. We discuss several methods for
determining the PMC scale for QCD processes. We show that a single global PMC
scale, valid at leading order, can be derived from basic properties of the
perturbative QCD cross section. The elimination of the renormalization scale
ambiguity and the scheme dependence using the PMC will not only increase the
precision of QCD tests, but it will also increase the sensitivity of collider
experiments to new physics beyond the Standard Model.
