  The value of the $V_{ud}$ matrix element of the Cabibbo-Kobayashi-Maskawa
(CKM) matrix can be derived from nuclear superallowed beta decays, neutron
decay, and pion beta decay. We survey current world data for all three. Today,
the most precise value of $V_{ud}$ comes from the nuclear decays; however, the
precision is limited not by experimental error but by the estimated uncertainty
in theoretical corrections. The neutron data are approximately a factor of four
poorer in precision but this could change dramatically in the near future as
planned experiments come to fruition. The nuclear result (and the most recent
of the neutron decay results) differ at the 98% confidence level from the
unitarity condition for the CKM matrix. We examine the reliability of the small
calculated corrections that have been applied to the data, and assess the
likelihood of even higher quality nuclear data becoming available to confirm or
deny the discrepancy. Some of the required experiments depend upon the
availability of intense radioactive beams. Others are possible today.
