The quenching behavior of coal dust-air flames was investigated. The literature relevant to coal dust-air flames was reviewed, including information on the agglomeration, sedimentation, and dispersal of dust clouds. The simple equations relating the various concentrations involved in dust flame propagation are given, and their implications are elaborated. It is shown that the concentrations seen by a moving flame in a sedimenting dust cloud can differ significantly from the concentration commonly measured in such experiments. The known dust cloud phenomena that appeared significant in flame propagation studies were experimentally evaluated, and quenching diameters, flame speeds, and lean propagation limits for several tube diameters were measured. For Pittsburgh seam coal, the minimum, quenching diameter (mqd) for dust between 10 and 20 um was found to be between 0.7 and 0.8 Cm, and the lean propagation for 3.2-Cm tubes was found to be approximately 200 mg/l. Similar mqd data were also obtained for Pocahontas and sewell coals, beulah lignite, and a 50-gal/ton oil shale. The effects of temperature, coal particle size, and propagation tube material were also investigated.