Weak-localization effects, especially the dependence of the resistance on a magnetic field, have become a unique probe for determining characteristic times for conduction electrons in metals. Analysis of the magnetoresistance data is usually made with the theory of Hikami et al., which is derived in the model of free electrons and s-type impurity scattering. We show that the results of Hikami et al. are universally valid for electrons with an arbitrarily complex Fermi surface (anisotropic, several bands) and arbitrary impurity scattering (anisotropic, interband scattering). The complex band-structure and scattering properties enter only in the resistivity, the electronic density of states, and properly averaged lifetimes, which can be obtained from a magnetoresistance experiment. © 1985 The American Physical Society.