Laser-based diagnostic methods can measure the absolute concentrations of various atoms and molecules, even at low concentrations. They can also measure temperature with high accuracy. One very important advantage of these methods is the ability to make measurements without affecting the studied process, for example flame. Laser-based spectroscopy can thus provide important information for the validation of chemical mechanisms in well-defined, low-pressure flat flames. This paper reviews several methods: laser-induced fluorescence, cavity ring-down spectroscopy, intracavity laser absorption spectroscopy, and polarization spectroscopy. All four are sufficiently sensitive to measure the concentrations of trace species in low-pressure flat flames. The complementarity, of the methods for combustion diagnostics is also discussed. (C) 2009 Elsevier Ltd. All rights reserved