An account is given of the theory of magnetophonon resonanc--an effect arising from the inelastic character of the scattering of electrons by optical phonons in strong magnetic fields. The electron-phonon collision frequency depends nonmonotonically on the magnetic field: It increases sharply when the optical-phonon energy $\hbar\omega_0$ becomes a multiple of the cyclotron energy $\hbar\Omega$. Therefore, all kinetic coefficients of a dissipative nature should have an oscillatory dependence on the magnetic field. The experimental study of the magnetophonon oscillations makes it possible to determine the cyclotron mass of the electrons, and also its dependence on temperature, pressure, etc. Thanks to development in technique for high magnetic fields, magnetophonon resonance has become both one of the most important methods for determining the band-structure parameters of semiconductors and a means for studying the interaction of electrons with optical phonons.