A unified theory of transition-metal impurities in a superconductor is presented, which employs the Hartree-Fock approximation and covers the nonmagnetic and magnetic cases. It is an application of Anderson’s theory [Phys. Rev. 124 (1961), 41] to superconducting host metals. The influence of transition-metal impurities on superconductivity is examined with special attention to the existence of the localized excited state in the energy gap, the transition temperature, the density of states, the jump of the specific heat at the transition temperature, the critical magnetic field, etc., and the experimental results on Zn-transition-metal alloys are compared with the theory. Relation to the Zittartz-Muller-Hartmann theory is briefly discussed.