Magnetic methods are widely used in exploration, engineering, borehole and global geophysics, and the subjects of this book are the physical and mathematical principles of these methods regardless of the area of application. Beginning with Ampere's law the force of interaction between currents is analyzed, and then the concent of the magnetic field is introduce and the fundamental features are discussed. Special attention is paid to measurements of relaxation processes, including topics as the spin echoes or refocusing. Also the special role of the magnetic method in the development of the plate tectonic theory is described.