Quantum noise limits the sensitivity of both high-frequency and low-frequency gravitational wave detectors. This results from the quantum nature of the light arriving at the detector and mirrors. In this Chapter, the theoretical treatment of the quantization of light is described as well as its different states: Fock states, coherent states and squeezed states. In addition, the quantum theoretical description of an interferometer and of quantum noise is given, concluding explaining how squeezed states can be used to reduce the quantum noise.