  The theoretical statements about a restoration of a superconductivity at
magnetic fields higher than the quasiclassical upper critical field and a
reentrance of superconductivity at temperatures $T_c(H)\approx T_c(0)$ in the
superconductors with open Fermi surfaces are reinvestigated taking into account
a scattering of quasiparticles on the impurities.
  The system of integral equations for determination of the upper critical
field parallel to the conducting planes in a layered conventional and
unconventional superconductors with impurities are derived. The $H_{c2}(T)$
values for the "clean" case in the Ginzburg-Landau regime and at any
temperature in the "dirty" case are found analytically. The upper limit of the
superconductor purity when the upper critical field definately has a finite
value is established.

Superconductivity