Universality of the weak interaction

One test of the universality of weak interactions is to demonstrate that the Cabibbo-Kobayashi-Maskawa matrix is unitary. The first-row test that the sum of the squares of the first-row elements add to one requires precision experiments in nuclei, neutrons, pions and kaons and precision calculations of symmetry-breaking and radiative corrections. This talk will give the current status of the first-row test and elaborate on the theoretical corrections.