Precision measurements and CKM unitarity

Determinations of vertical bar Vud vertical bar and vertical bar Vus vertical bar along with their implications for the unitarity test vertical bar Vud vertical bar 2+vertical bar Vus vertical bar2+vertical bar Vub vertical bar2=1 are discussed. The leading two-loop radiative corrections to neutron β decay are evaluated and used to derive a refined relationship vertical bar Vud vertical bar2τn(1+3gA2)=4908(4) s. Employing vertical bar Vud vertical bar=0.9740(5) from superallowed nuclear decays and the measured neutron lifetime τn=885.7(7) s leads to the precise prediction gA=1.2703(8) which is compared with current direct experimental values. Various extractions of vertical bar Vus vertical bar are described and updated. The long accepted Particle Data Group value of vertical bar Vus vertical bar from fitted Ke3 decay rates suggests a deviation from CKM unitarity but it is contradicted by more recent experimental results which confirm unitarity with good precision. An outlook for possible future advances is given