  A brief review of works on the microscopic theory of determining the upper
critical field in two-band isotropic and anisotropic superconductors is given.
The research is based on a set of the Ginzburg-Landau equations for the order
parameters in a magnetic field that are studied in terms of the classical
approach to a superconducting system in a magnetic field. Two inequivalent
energy bands with different topology of Fermi surface cavities overlapping on
the Fermi surface are discussed. The cases of the direction of the external
magnetic field H\to// the (ab) plane and H\to // the crystallographic c axis
are studied. The equations for determining Hc2(ab) and Hc2(c) for a pure
superconductor and a superconductor doped with electrons and holes are derived.
The analytical solutions to these equations in the vicinity of the
superconducting transition temperature (Tc - T<<Tc) and in the vicinity of zero
(T<<Tc) are found. The temperature and impurity dependences of the upper
critical fields Hc2(ab) and Hc2(c), as well as the anisotropy coefficient
\gammaH, are studied. The resulting theory is applied to determine the
dependences of the above magnetic characteristics of intermetallic compound
MgB2. The theory agrees qualitatively with experimental data.
