Fundamentals of BCS and GLAG theories of superconductivity are reviewed with a focus on high.;,frequency properties of bulk superconductors, superconducting films and superconducting cavities. Superconductivity as a macroscopic quantum coherent state. Supercurrents and persistent currents. Condensate and excitations. Complex penetration depth and RF losses. Mechanisms of Q degradation in superconducting cavities at increasing a.c. field amplitude. Depairing effects and vortex nucleation mechanisms. Surface superconductivity and tilted vortices. Material parameters and factors responsible for ultimate performance of superconducting resonators.