Impurity scattering in superconductors

We present some of the recent theoretical studies on the impurity effects in conventional superconductors, such as magnetic and ordinary impurity effects in dirty and weak localization limits, and describe successfully unexplained experimental results. We find that the critical sheet resistance for the suppression of superconductivity in thin films depends on superconductor, and point out that impurity dopings in high T_c superconductors cause a metal-insulator transition and thereby suppress T_c.