Kubo, T. Gurevich, A.V. A Unified Theory of Surface Resistance and the Residual Resistance of SRF Cavities at Low Temperatures JACoW Geneva, Switzerland The Mattis-Bardeen (MB) theory has been widely used for the calculation of the surface resistance of superconductors at weak RF fields. It is well known that the observed surface resistance decreases exponentially as the temperature decreases but tends to a constant residual resistance which is not described by the MB theory. Using the quasiclassical formalism of the microscopic BCS theory, we have developed a unified theory of the surface resistance which, in addition to the MB contribution, includes the residual resistance. Based on this theory, possible materials treatments to reduce the surface resistance are discussed.