This paper discusses spin excitation effects in superconductors with an emphasis on experiment. First, a review is presented of the superconductivity in dilute alloy systems which exhibit localized spin fluctuations associated with resonant impurity states. It is shown that when the localized spin fluctuation temperature is much larger than superconducting temperatures; (1) the transition temperature falls off rapidly with impurity concentration following a modified exponential, and (2) the thermodynamic properties (e.g., the specific heat jump at the transition and the critical field) obey the BCS law of corresponding states. A comparison is made to a recent theory by Kaiser of superconductivity in the presence of nonmagnetic resonant impurity states. Next, experiments on La and La intermetallic compounds with Ce impurities which simultaneously exhibit superconductivity and the Kondo effect are reviewed. Finally, pressure‐induced magnetic‐nonmagnetic transitions of Ce impurities in superconductors are discussed and compared with certain theoretical models.