The chapter begins with the discovery of the Zeeman effect and covers the historical development of nuclear magnetic resonance (NMR), both theory and experiment. Phenomena such as motional and exchange narrowing of absorption peaks in nuclear magnetic resonance are discussed. It also presents the experimental and theoretical aspects of nuclear quadrupole resonance, electron paramagnetic resonance, ferromagnetic and antiferromagnetic resonances, spin-wave resonance, perturbed angular correlation, and the Mössbauer effect. Concepts like hyperfine interactions and the Knight shift are explained in the context of their influence on resonance spectra.