This lecture covers the quantum optics of resonators, focusing on the cavity Hamiltonian and the frequency operator. It discusses the quantum Langevin equations for forward and backward evolution, as well as the Purcell effect in spontaneous emission. Examples and applications are also presented, including cavity-enhanced single-atom spontaneous emission and the observation of inhibited spontaneous emission. The lecture delves into cavity quantum electrodynamics, comparing resonant and non-resonant cases, and explores the input-output relation in cavity transmission and reflection.