The physics of gas discharge lamps is reviewed, with particular emphasis on new developments in the field. In addition to the most important light sources (fluorescent and high-pressure discharge lamps), a number of less common sources are also discussed. The review covers not only the physics of the discharges themselves, but also their interactions with lamp materials and external driving circuits. Important historical steps in lamp product development are described and special sections are devoted to the properties of light, diagnostics, and modeling of the discharges. A detailed, but not exhaustive, review of the published literature is provided and some possible future directions for research in the subject are discussed.