  For many decades, the main source of information on the top-left corner
element of the Cabibbo-Kobayashi-Maskawa quark mixing matrix $V_{ud}$ were
superallowed nuclear beta decays with an impressive 0.01\% precision. This
precision, apart from experimental data, relies on theoretical calculations in
which nuclear structure-dependent effects and uncertainties play a prime role.
This review is dedicated to a thorough reassessment of all ingredients that
enter the extraction of the value of $V_{ud}$ from experimental data. We tried
to keep balance between historical retrospect and new developments, many of
which occurred in just five past years. They have not yet been reviewed in a
complete manner, not least because new results are a-coming. This review aims
at filling this gap and offers an in-depth yet accessible summary of all recent
developments.
