The quest for better superconductors, with higher transition temperatures that perform at higher magnetic fields and lead to higher critical currents is one of the major challenges in condensed matter physics. In the Bardeen–Cooper–Schrieffer theory of superconductivity, electrons form Cooper pairs as a result of an interaction mediated by vibrations in the underlying crystal structure (phonons). In my talk I will review different experimental techniques which allow an evaluation of the strength of the interaction between electrons and phonons. The theoretical constraints for an unconventional superconductivity caused by repulsive interaction will be also discussed.