This review summarizes studies of the energy gap and selected electronic properties of CuO₂ superconductors. The main focus is placed on results of inelastic light scattering experiments starting as early as 1987 just a year after the discovery of superconductivity in copper–oxygen compounds by Georg Bednorz and Alex Muller. I add a few historical and personal remarks on superconductivity in general and on the contribution of the eminent scientist Karl Alex Muller who passed away in January 2023.