  We analyze the prospective impact of supersymmetric radiative corrections on
tests of charged current universality involving light quarks and leptons.
Working within the R-parity conserving Minimal Supersymmetric Standard Model,
we compute the corresponding one-loop corrections that enter the extraction of
the Cabibbo-Kobayashi-Maskawa matrix element $V_{ud}$ from a comparison of the
muon-decay Fermi constant with the vector coupling constant determined from
nuclear and neutron $\beta$-decay. We also revisit earlier studies of the
corrections to the ratio $R_{e/\mu}$ of pion leptonic decay rates $\Gamma[\pi^+
\to e^+ \nu (\gamma)]$ and $\Gamma[\pi^+ \to \mu^+ \nu (\gamma)]$. In both
cases, we observe that the magnitude of the corrections can be on the order of
$10^{-3}$. We show that a comparison of the first row CKM unitarity tests with
measurements of $R_{e/\mu}$ can provide unique probes of the spectrum of first
generation squarks and first and second generation sleptons.
