Abstract. Laser-induced fluorescence (LIF) is a method of considerable utility for the measurement of the transient free radicals which are the keys to the chemistry of flames. Collisions experienced by the electronically excited state can alter the magnitude and the spectral form of the fluorescence signals. Recent studies on both quenching and energy transfer collisions, and their influence on LIF measurements, are treated in this review; special emphasis is given to the important and popular OH molecule. Different solutions to the problem of accounting for quenching are considered, and both effects and exploitation of energy transfer within the excited state are discussed. Although further research is needed to better quantify these collisional effects, LIF can currently provide data significant for the understanding of combustion chemistry.