We study the dependence of quark condensate $\Sigma$ on an external magnetic field. For weak fields, it rises linearly with the field. Pion mass and residue are also shifted so that the Gell-Mann - Oakes - Renner relation is satisfied. In the strong field region, $\Sigma(H) \propto (eH)^{3/2}$.Comment: 13 pages LaTeX, 5 postscript files for figure

The low-energy QCD theorems are generalized in the presence of a constant magnetic field H. Two-loop approximation for the vacuum energy density in the framework of the chiral perturbation theory was obtained and the quark and gluon condensates were found as the functions of H.

In the first order of chiral perturbation theory the corrections to F π 0 and M π 0 in a magnetic field at finite temperature have been found. It was shown that they are shifted in such a manner that Gell-Mann -Oakes -Renner relation remains valid under these conditions.

The low energy theorems for QCD are generalized to finite temperature and chemical potential, including non-zero quark masses.