This lecture covers the fundamental concepts of photochemistry, including the kinetics of excited state's radiative decay, Jablonski's diagram, spin correlation effect, non-radiative and radiative deactivation processes, Stokes shift, emission spectrum, solvation dynamics, and kinetic competition of deactivation processes. It also discusses the selection rules for non-radiative processes, spin-orbit coupling, and quenching of the emission, providing insights into the intricate processes governing photochemical reactions.