  This article surveys the important questions attached to unitarity tests of
the Cabibbo-Kobayashi-Maskawa matrix, with a focus on the direct determinations
of the magnitude of CKM matrix elements. The current CKM anomalies are
discussed and the clear-cut prospects at LHCb and Belle II envisaged. The
anticipated precision on $C\!P$-conserving and $C\!P$-violating observables at
the projected upgrades of the LHCb and Belle~II experiments is examined from
the point of view of the search for New Physics in $B$-meson mixing, addressing
the bottlenecks in precision for the interpretation of the measurements for
future experiments and highlighting the related theoretical and experimental
challenges. The vibrant prospects at future $e^+e^-$ colliders running at the
$Z^0$ pole and $W^+W^-$ are eventually discussed.
