This book is devoted to a quasi-classical treatment of quantum transitions, with an emphasis on nuclear magnetic resonance, nuclear quadrupole resonance and electric dipolar resonance. The method described here is based on the quasi-classical description of condensed matter, and makes use of the equation of motion of harmonic oscillators with external forces. In addition to known results in magnetic resonance, the book also presents parametric resonance for electric dipoles and dipolar interaction which may lead to spontaneous electric polarization.