  Nuclear super-allowed $\beta$ decay has been used to obtain tight limits on
the value of the CKM matrix element $V_{ud}$ that is important for unitarity
tests, and therefore important for tests of the standard model. Current
requirements on precision are so intense that effects formerly thought too
small to matter are now relevant. This article is a brief review of personal
efforts to include the effects of strong interactions on Fermi $\beta $ decay.
First I examine the role of isospin violation in the decay of the neutron. The
size of the necessary correction depends upon detailed strong-interaction
dynamics. The isospin violating parts of the nucleon wave function, important
at the low energy of $\beta$ decay, can be constrained by data taken at much
higher energies, via measurements, for example, of $ed\to e'\pi^\pm +\rm X $
reactions at Jefferson Laboratory. The next focus is on the role of nuclear
short-ranged correlations, which affect the value of the correction needed to
account for isospin violation in extracting the value of $V_{ud}$. The net
result is that effects previously considered as irrelevant have become relevant
for both neutron and nuclear $\beta$ decay.
