Planar laser-induced fluorescence (PLIF) is a non-intrusive technique for measuring scalar concentrations in fluid flows. A fluorescent dye is used as a scalar proxy, and local fluorescence caused by excitation from a thin laser sheet can be related to dye concentration. This review covers quantitative PLIF in aqueous flows, with discussions of fluorescence theory, experimental methods and equipment, image processing and calibration, and applications of the technique.