Publisher Summary This chapter describes ideas that affect the theoretical aspects of solar cell efficiencies. The overestimate of the efficiencies obtained is due to the neglect of radiative and non-radiative recombination. This leads to the loss of electron-hole pairs that can therefore, not contribute to the current. This effect is essentially due to electronic energy being given to the lattice and energy transfer to other electrons by the Auger effect. Higher efficiencies have been explored by having several materials with decreasing energy gap in series. These are the tandem cells, which are at most four or five in series. The theory for an infinite number has also been worked out. This procedure makes use of the high-energy photons.