We analyze in detail how the scattering by nonmagnetic impurities affects the shape and amplitude of the order parameter ͑OP͒ and the density of states in anisotropic superconductors in the framework of BCS theory. Special attention is paid to the case when the OP is a mixture of d and s waves changing its sign on the Fermi surface. The critical temperature is shown to decay with the increase of the residual resistance according to the power law. At zero temperature impurity scattering gives rise to a peculiar phase transition from a gapless regime to a state with a finite gap in the quasiparticle spectrum. ͓S0163-1829͑96͒02342-9͔

It is shown that layered superconductors are subjected to a phase transition at zero temperature provided the order parameter (OP) reverses its sign on the Fermi-surface but its angular average is finite. The transition is regulated by an elastic impurity scattering rate 1/τ . The excitation energy spectrum, being gapless at the low level of scattering, develops a gap as soon as the scattering rate exceeds some critical value of 1/τ ⋆ .

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A microscopic theory of the plasma resonance in layered metals is presented. It is shown that electron-impurity scattering can suppress the plasma resonance in the normal state and sharpen it in the superconducting state.