A review. This paper reviews with refs. the fundamental concepts for how to devise and apply quant. measurement techniques for studies of fuel concn., temp., 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 quant. 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 org. mols. is presented in an overview followed by discussions on specific details of the temp.-, pressure-, and mixt.-dependence of the laser-induced fluorescence strength of aliph. ketones, like acetone and 3-pentanone, and toluene. Models that describe the fluorescence are discussed and evaluated with respect to their functionality. Examples for quant. applications are categorized in order of increased complexity. These examples include simple mixing expts. under isothermal and isobaric conditions, fuel/air mixing in engines, temp. measurements, and mixing studies where fuel and oxygen concns. vary. A brief summary is given on measurements of fuel concns. in multiphase systems, such as laser-induced exciplex spectroscopy. Potentially adverse effects that added tracers might have on mixt. 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 quant., tracer-based LIF measurements.