  We discuss our present knowledge of $\alpha_s$, the fundamental running
coupling or effective charge of Quantum Chromodynamics (QCD). A precise
understanding of the running of $\alpha_s(Q^2) $ at high momentum transfer,
$Q$, is necessary for any perturbative QCD calculation. Equally important, the
behavior of $\alpha_s$ at low $Q^2$ in the nonperturbative QCD domain is
critical for understanding strong interaction phenomena, including the
emergence of mass and quark confinement. The behavior of $\alpha_s(Q^2)$ at all
momentum transfers also provides a connection between perturbative and
nonperturbative QCD phenomena, such as hadron spectroscopy and dynamics. We
first sketch the origin of the QCD coupling, the reason why its magnitude
depends on the scale at which hadronic phenomena are probed, and the resulting
consequences for QCD phenomenology. We then summarize latest measurements in
both the perturbative and nonperturbative domains. New theory developments
include the derivation of the universal nonperturbative behavior of
$\alpha_s(Q^2)$ from both the Dyson-Schwinger equations and light-front
holography. We also describe theory advances for the calculation of gluon and
quark Schwinger functions in the nonperturbative domain and the relation of
these quantities to $\alpha_s$. We conclude by highlighting how the
nonperturbative knowledge of $\alpha_s$ is now providing a parameter-free
determination of hadron spectroscopy and structure, a central and long-sought
goal of QCD studies.
