  We summarize our recent results on the description of the evolution of
Majorana neutrinos in external fields. First we discuss the purely quantum
method which involves the secondly quantized Weyl spinors. In frames of this
formalism we find exact solutions of the classical Hamilton equations for Weyl
fields in background matter, which then are used to canonically quantize the
system. The analog of the effective Hamiltonian for the description of
spin-flavor oscillations of Majorana neutrinos in matter and a magnetic field
is also obtained. Finally we discuss another approach for the treatment of
Majorana neutrinos in external fields, which is based on the relativistic
quantum mechanics. The latter method does not require the quantization of the
neutrino wave function. Nevertheless we demonstrate that the relativistic
quantum mechanics approach also appropriately describes spin-flavor
oscillations of relativistic Majorana neutrinos.
