Quantum mechanics, despite its abstract and unintuitive nature, is increasingly used in real-life applications. This thesis explores the current status of cavity quantum electrodynamics and its role in applications of quantum optics to quantum technologies. Various approaches to the treatment of optical cavities are discussed, with particular focus on a treatment in terms of cavities as linear optical devices, with a nonlinearity introduced by an atom, molecule, quantum dot etc in the cavity. Open quantum systems such as optical cavities coupled to a free external radiation field can be described by a quantum master equation. This thesis develops a description of such a system in which the Hamiltonian describes the coherent evolution of light inside the cavity, and the damping term describes the leaking of light out of the cavity mirrors. The goal of this approach is to describe couple networks of optical cavities in which information is transferred across the network coherently.