  The importance of the isospin symmetry and its breaking in elucidating the
properties of atomic nuclei is reviewed. The quark mass splitting and the
electromagnetic origin of the isospin symmetry breaking (ISB) for nuclear
many-body problem is discussed. The experimental data on isobaric analogue
states cannot be described only with the Coulomb interaction, and ISB terms in
the nucleon-nucleon interaction are needed to discern the observed properties.
In the present work, the ISB terms are explicitly considered in nuclear energy
density functional and spherical shell model approaches, and a detailed
investigation of the analogue states and other properties of nuclei is
performed. It is observed that isospin mixing is largest for the $N=Z$ system
in the density functional approach.
