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.

We shall first discuss the origin of the spins and magnetic dipole moments of the nucleons and nuclei. The nuclear magnetic resonance (NMR) lineshape will then be reviewed in general theoretical terms first, before turning to the microscopic sources of line broadening and frequency shifts that are valid for solid materials only. The relaxation mechanisms of nuclear spins will then be described, focusing on relaxation via paramagnetic electrons. During frozen spin operation the polarization loss is different for positive and negative polarization, which is explained by the polarization-dependent heat transfer from the nuclear spins to the liquid helium coolant.