  We provide sufficient conditions for the discreteness of spectrum for
magnetic Schr\"odinger operators. They generalize the classical result by
K.Friedrichs (1934) and earlier results by J.Avron, I.Herbst and B.Simon
(1978), A.Dufresnoy (1983) and A.Iwatsuka (1990) which were obtained in the
absence of electric field. Our conditions are formulated as conditions of
growth at infinity for some effective potentials. More precise sufficient
conditions are formulated by use of the Wiener capacity and extend earlier work
by A.M.Molchanov (1953) and V.Kondratiev and M.Shubin (1999) who considered the
case of the operator without magnetic field. These conditions become necessary
and sufficient in case when there is no magnetic field and the electric
potential is semi-bounded below.
