Abstract
Laser induced fluorescence (LIF) has become a common detector of atoms and molecules allowing to determine their internal state distributions. In this paper we mention the advantages of both kinds of lasers, cw or pulsed. We review some aspects of the LIF process, such as saturation or polarization effects, which could alter the results if they were not taken into account. We also indicate how LIF can be used to measure relaxation times and thus rate constants of the relaxation processes: some experimental results obtained in our laboratory illustrate these points.