We shall first outline the types of interactions of spins, which are most important for solid polarized targets: the magnetic dipole interaction, the quadrupole interaction, the spin-orbit interaction and the hyperfine interaction. Other direct and indirect spin interactions are described: these give rise to the chemical shift, the Knight shift, molecular spin isomers and to the exchange interaction of electron pairs. These, and in particular the dipolar interaction, are then used in the discussion of the magnetic resonance phenomena, such as the resonance line shape and saturation. The magnetic resonance absorption and the transverse susceptibility are discussed starting from the first principles, and Provotorov's equations are derived. The relaxation of spins, which is phenomenologically introduced already for the saturation, is then overviewed in greater depth, before closing with sudden and adiabatic changes of spin systems in the rotating frame.