Applications of mercury in lighting involve the use of pure liquid and various amalgams. Mercury is used primarily in fluorescent, high-pressure sodium and metal halide lighting. The operation and construction of these and other lamps are explained. The thermodynamics of the amalgams used in fluorescent and high-pressure sodium lamps is discussed. Extensive data on the vapor pressure of various fluorescent lamp amalgams are tabulated or displayed graphically. Properties of binary sodium–mercury and ternary sodium–mercury amalgams are outlined in significant detail. A short discussion of experimental methods for measuring the mercury vapor pressure of fluorescent lamp amalgams is given.