A review is presented of the theoretical and experimental papers on spin resonance, in which the concept of the reservoir of spin-spin interactions in a solid is used. We explain the main ideas connected with the introduction of two spin temperatures, $T_Z$ and $T_{SS}$, to describe quasi-equilibrium in the Zeeman and spin-spin subsystems (Provotorov's theory); methods of strongly altering $T_{SS}$ are indicated. We consider the effects due to the shift of the $T_{SS}$ following saturation of the ESR line, dynamic polarization and spin-lattice relaxation of the nuclei, and electron cross relaxation both between different lines and inside lies with inhomogeneous broadening. Experiments show that the concept of spin-spin temperature in ESR are well confirmed and explain many new phenomena, namely induced emission at not too strong saturation of the ESR lines; polarization, relaxation, and thermal mixing of the nuclear spins by contact with the reservoir of the electronic spin-spin interactions; limitation of the redistribution of the saturation over the ESR spectrum in the case of cross relaxation; stimulated spectral diffusion, etc. It is shown that the shift of $T_{SS}$ becomes equally manifest both in homogeneous ESR lines (with spin-spin broadening) and in inhomogeneous lines, provided only the spin-spin interactions are more effective than the spin-lattice ones.