  We discuss the recent progress in the study of semileptonic kaon and pion
decays, including new experimental results, improved electroweak radiative
corrections, form factor calculations and isospin-breaking effects. As a
result, we obtain $|V_{us}|=0.22309(40)(39)(3)$ from kaon semileptonic decays
and $|V_{us}/V_{ud}|=0.22908(66)(41)(40)(2)(1)$ from the ratio between the kaon
and pion semileptonic decay rates. We report an apparent violation of the
top-row Cabibbo-Kobayashi-Maskawa matrix unitarity at a $3.2\sim 5.6\sigma$
level, and a discrepancy at a $2.2\sigma$ level between the value of
$|V_{us}/V_{ud}|$ determined from the vector and axial charged weak
interactions. Prospects for future improvements in those comparative precision
tests involving $|V_{ud}|$, $|V_{us}|$ and their implications for physics
beyond the Standard Model are described.
