Fire spreads through the crowns of shrub lands such as chaparral and sagebrush; however, these fires were not considered crown fires by the fire modeling community which is focused on crown fires in forests. A series of experiments and model development, both physical and semi-empircal, by Forest Service scientists and their research partners are helping to advance an understanding of how fire spread in these important fuel types. The scientists report that factors such as wind, three-dimensional fuel characteristics such as bulk density, foliage location, and moisture content are important variables related to the successful propagation of a flame through the living shrub crown. Bulk density is important because it influences the ability of hot flame gases to flow through the shrub crown and ignite new fuel particles infoliage and branches. The semi-empirical model is able to produce reasonable results with short computational time and the physical model is able to provide detailed results of heat transfer and gas flow with much greater computational time.