This lecture covers the spin relaxation mechanisms including Korringa relation, Elliott-Yafet mechanism, Dyakonov-Perel mechanism, phonon-drag, electron-phonon interaction, electron-magnon, and spin mixing. It discusses the principles of spin relaxation, time-dependent perturbation in the Schroedinger equation, calculations involving the Hamiltonian of the system, relaxation rates, momentum conservation, phonon drag, electron-magnon interactions, and symmetries in the context of spin-orbit interactions.