The book is devoted to the description of the fundamentals in the area of magnetic resonance. The book covers two domains: radiospectroscopy and quantum radioelectronics. Radiospectroscopy comprises nuclear magnetic resonance, electron paramagnetic resonance, nuclear quadrupolar resonance, and some other phenomena. The radiospectroscopic methods are widely used for obtaining the information on internal (nano, micro and macro) structure of objects. Quantum radioelectronics, which was developed on the basis of radiospectroscopic methods, deals with processes in quantum amplifiers, generators and magnetometers. We do not know analogues of the book presented. The book implies a few levels of the general consideration of phenomena, that can be useful for different groups of readers (students, PhD students, scientists from other scientific branches: physics, chemistry, physical chemistry, biochemistry, biology and medicine).