A review is given of the present state of the theory of dynamic nuclear polarization (DNP) by the dipole electron-nucleus interaction in solids. The two-particle (solid effect), three-particle (electron-nucleus cross relaxation), and many-particle (dynamic cooling) mechanisms of DNP are discussed both for idealized conditions and under inhomogeneous broadening of the ESR line. The effect of foreign impurities, spin diffusion, phonon bottlenecks, and very low temperatures is analyzed. The treatment is based on a unified approach based on the concepts of spin temperature and thermal mixing in a rotating coordinate frame. The review covers experimental data illustrating the DNP mechanisms, possible applications of this phenomenon in the development of highly polarized proton targets, and so on. A brief discussion is also given of other DNP methods and mechanisms (the Overhauser effect and optical nuclear polarization).