The interaction of microwaves with superconductors was explored via extensive measurements of the surface resistanceRsand reactanceXsat 10 GHz of superconducting Sn. The measurements were carried out as functions of thicknessdand temperatureTfor Sn films ranging in thickness from 190 A˚ to bulk. By varying the thickness with accompanied variation of mean free path, we were able to examine the wave‐vector dependence of the microwave‐superconductor interaction. An analysis, based on the Bardeen–Cooper–Schrieffer electrodynamical kernel, was developed for calculatingRsandXsas functions ofd,T, and frequency. Details of the measurement techniques and analysis are presented. Experiment and theory agree excellently, with no adjustable parameters. For films withd<800 A˚ we find that the electrodynamics is local because of impurity scattering, while for thicker films nonlocal effects are important. The results imply that superconducting films with microwave response that agrees with theoretical expectations can be fabricated for potential applications in a variety of superconducting structures.