  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.
