Technical Abstract: Japanese brome (Bromus japonicus Thunb) and downy brome (B. tectorum L.), alien weedy cool-season annual grasses have invaded thousands of acres of the Northern Great Plains, Great Basin, California Annual Grasslands, and Palouse Prairie. A general discussion of research findings on particularly Japanese brome is presented. Abundance of brome is dependent upon availability of seed, amount and distribution of rainfall, temperature, and availability of soil nitrogen. More than 10,000 annual brome seeds can be present in a square yard in the mixed- grass prairie of the Northern Great Plains. Germination characteristics enhance the persistence of annual bromes on rangelands. Annual bromes add to the total forage base at the expense of perennial grasses (e.g., western wheatgrass), and presence of annual bromes can reduce livestock performance. Suppression of annual bromes is difficult. Annual bromes will likely persist on Northern Great Plains ranges. Land managers will have to decide if the annual bromes are a problem on their operations and then determine the most economical management alternatives.