The principle of laser-induced fluorescence (LIF) is first reviewed and the necessary hardware for the diagnostic is described. Following this is a review of the present state of the art, including what has so far been measured and discussed, and what is currently being developed. LIF is the technique in which atoms, molecules and ions are excited from low-lying energy levels to higher levels, and the resulting fluorescence observed when they decay back to the lower levels. Because spectral lines of different species very rarely overlap and the cross sections for the resonant excitations are large, the technique is very powerful for observing a particular species with very high sensitivity. This characteristic offers a unique feature for the understanding of particle behaviour in plasmas, and if Stark and Zeeman effects are large enough to be detectable, for measurements of electric and magnetic fields. The key hardware for the technique is a tunable laser, whose wavelength is tuned to transitions of the species to be detected. Various tunable lasers are described, combined with techniques for narrowing and scanning of the laser output spectrum for spectral profile measurements.