Abstract: This note reviews the role of superallowed transitions in determining the strength of the weak interaction among the lightest quarks and in searching for new physics beyond the standard electroweak model. The two sets of superallowed decays in nuclei considered here are pure Fermi and mirror transitions. The first have been scrutinized for more than 50 years. The most relevant results are presented and the role of the nucleus-dependent radiative correction and nucleus-independent inner radiative correction are reviewed. In this context, the systematic study of mirror transitions started about 15 years ago. Despite the significant progress made since then, the data is still limited by experimental uncertainties. Combining the results from all superallowed transitions, which are fully consistent, provides a test of unitarity of the first row of the Cabibbo-Kobayashi-Maskawa matrix, which displays a 2 sigma tension with the standardmodel.Superallowed transitions in beta decay are considered to be the “cleanest” ones in terms of hadronic contributions arising from the nuclear medium. These transitions have been identified since the early days in the study of beta decay and have played a crucial role in determining the strength of weak processes involving the lightest u and d quarks. They offer today a sensitive window to search for NP through high precision measurements. This paper reviews the contributions of pure Fermi and mirror superallowed transitions, to determine parameters within the SMor to constrain NP. It relies in particular on the results of four recent reviews and global analyses which can be found in Refs. [1-4]. Although neutron decay is the simplest mirror transition, the recent progress in neutron decay is not covered here besides mentioning the most relevant results.