All substances can be classified as diamagnetic and paramagnetic. The latter contain elementary magnetic moments, which can be oriented by magnetic field, what leads to magnetization in the direction of the field. Magnetic moments in dielectrics and semiconductors can be regarded as localized on crystal-lattice sites. In metals, the magnetic moments of 3d and 4f shells are only partly localized (to a greater degree, in 4f metals), and the delocalized magnetic-moment density of conduction electrons is to be taken into account. The oscillations of magnetization are inevitably accompanied by dissipation of their energy, which is transformed into other kinds of energy, mainly, into the thermal energy. In particular, the Bloch equation with two relaxation times, which is commonly used for paramagnetic resonance, has been applied to ferromagnetic resonance, too. However, the processes of energy dissipation in ferromagnets, being numerous and complicated, cannot be described strictly by an equation of motion either with one or with two dissipation parameters.