  Superallowed $0^+ \to 0^+$ nuclear beta decay provides a direct measure of
the weak vector coupling constant, $\GV$. We survey current world data on the
nine accurately determined transitions of this type, which range from the decay
of $^{10}$C to that of $^{54}$Co, and demonstrate that the results confirm
conservation of the weak vector current (CVC) but differ at the 98% confidence
level from the unitarity condition for the Cabibbo-Kobayashi-Maskawa (CKM)
matrix. We examine the reliability of the small calculated corrections that
have been applied to the data, and conclude that there are no evident defects
although the Coulomb correction, $\delta_C$, depends sensitively on nuclear
structure and thus needs to be constrained independently. The potential
importance of a result in disagreement with unitarity, clearly indicates the
need for further work to confirm or deny the discrepancy. We examine the
options and recommend priorities for new experiments and improved calculations.
Some of the required experiments depend upon the availability of intense
radioactive beams. Others are possible with existing facilities.
