Precision measurements in allowed nuclear beta decays and neutron decay are reviewed and analyzed both within the Standard Model and looking for new physics. The analysis incorporates the most recent experimental and theoretical developments. The results are interpreted in terms of Wilson coefficients describing the effective interactions between leptons and nucleons (or quarks) that are responsible for beta decay. New global fits are performed incorporating a comprehensive list of precision measurements in neutron decay, superallowed 0$^{+}$→ 0$^{+}$ transitions, and other nuclear decays that include, for the first time, data from mirror beta transitions. The results confirm the V-A character of the interaction and translate into updated values for V$_{ud}$ and g$_{A}$ at the 10$^{−4}$ level. We also place new stringent limits on exotic couplings involving left-handed and right-handed neutrinos, which benefit significantly from the inclusion of mirror decays in the analysis.