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A brief review of works on the microscopic theory of determining the uppercritical field in two-band isotropic and anisotropic superconductors is given.The research is based on a set of the Ginzburg-Landau equations for the orderparameters in a magnetic field that are studied in terms of the classicalapproach to a superconducting system in a magnetic field. Two inequivalentenergy bands with different topology of Fermi surface cavities overlapping onthe Fermi surface are discussed. The cases of the direction of the externalmagnetic field H\to// the (ab) plane and H\to // the crystallographic c axisare studied. The equations for determining Hc2(ab) and Hc2(c) for a puresuperconductor and a superconductor doped with electrons and holes are derived.The analytical solutions to these equations in the vicinity of thesuperconducting transition temperature (Tc - T<<Tc) and in the vicinity of zero(T<<Tc) are found. The temperature and impurity dependences of the uppercritical fields Hc2(ab) and Hc2(c), as well as the anisotropy coefficient\gammaH, are studied. The resulting theory is applied to determine thedependences of the above magnetic characteristics of intermetallic compoundMgB2. The theory agrees qualitatively with experimental data.

The Upper Critical Field Hc2 in Advanced Superconductors with Anisotropic Energy Spectrum