pExperimental methods employing spin resonance effects nuclear magnetic resonance and electron spin resonance are broadly used in molecular science due to their unique potential to reveal mechanisms of molecular motion, structure, and interactions. The developed techniques bring together biologists investigating dynamics of proteins, material science researchers looking for better electrolytes, or nanotechnology scientists inquiring into dynamics of nano-objects. Nevertheless, one can profit from the rich source of information provided by spin resonance methods only when appropriate theoretical models are available. The obtained experimental results reflect intertwined quantum-mechanical and dynamical properties of molecular systems, and to interpret them one has to first understand the quantum-mechanical principles of the underlying processes.ppThis book concentrates on the theory of spin resonance phenomena and the relaxation theory, which have been discussed from first principles to