The safe, clean, and reliable operation of combustion devices depends to a large degree on the exact control of the fuel/air mixing process prior to ignition. Therefore, quantitative measurement techniques that characterize the state of the fresh gas mixture are crucial in modem combustion science and engineering. This paper presents the fundamental concepts for how to devise and apply quantitative measurement techniques for studies of fuel concentration, temperature, and fuel/air ratio in practical combustion systems, with some emphasis on internal combustion engines. The paper does not attempt to provide a full literature review of quantitative imaging diagnostics for practical combustion devices; rather it focuses on explaining the concepts and illustrating these with selected examples. These examples focus on application to primarily gaseous situations. The photophysics of organic molecules is presented in an overview followed by discussions on specific details of the temperature-, pressure-, and mixture-dependence of the laser-induced fluorescence strength of aliphatic ketones, like acetone and 3-pentanone, and toluene. Models that describe the fluorescence are discussed and evaluated with respect to their functionality. Examples for quantitative applications are categorized in order of increased complexity. These examples include simple mixing experiments under isothermal and isobaric conditions, fuel/air mixing in engines, temperature measurements, and mixing studies where fuel and oxygen concentrations vary. A brief summary is given on measurements of fuel concentrations in multiphase systems, such as laser-induced exciplex spectroscopy. Potentially adverse effects that added tracers might have on mixture formation, combustion, and the faithful representation of the base fuel distribution are discussed. Finally, a brief section describes alternative techniques to tracer-based measurements that allow studies of fuel/air mixing processes in practical devices. The paper concludes with a section that addresses key issues that remain as challenges for continued research towards the improvement of quantitative, tracer-based LIF measurements. (C) 2004 Elsevier Ltd. All rights reserved.