The main purpose of this publication is to summarize the most important aspects of fire behavior as we now know them. The author recognizes that there are still many unknowns in the behavior of forest and range fires. These unknowns will be the targets of future research. In the meantime it is important that the best available information on fire behavior be placed in the hands of the men who must carry on the vital task of fire control. In many respects this is a progress report on fire behavior research. A large part of the publication interprets the significance of past research results to an understanding of how fires start and spread under various forest conditions. Other parts of the publication reflect the results of recent research. Some of the information on fuel classification, burning index variations, topographic influences, and rate-of-spread estimates is reported for the first time. Four major phases of fire research at the Northern rocky Mountain Forest and Range Experiment Station provided basic knowledge which makes this publication possible. The development of methods for measuring fire weather and rating forest flammability with a burning index meter fostered systematic evaluation of weather influence on fire behavior. The fuel classification system provided an important basis for analyzing the rate of spread of fires. Studies of fire weather at various positions and aspects at the Priest River Experimental Forest contributed essential information on topographic influences. Detailed analysis of thousands of reports of actual fires uncovered many key facts and provided a means of testing several basic hypotheses on fire behavior.