Invasive annual grasses are one of the primary causes of conversion to novel ecosystems in the arid and semi-arid regions of the western US and are increasingly a global threat. In this Organized Oral Session, we focus on exotic brome-grasses which are among the most widespread and abundant, and thus best studied, invasive grasses. The presentations provide key analyses and information on the causes, consequences, and management alternatives for the most problematic of these species. Highlighted information follows. Invasion of exotic brome-grasses is typically initiated by anthropogenic disturbances related to land uses, like overgrazing by livestock, and development, such as urban expansion, oil and gas drilling, mining, and their related infrastructure. Importantly, land use and invasive species policies also play a significant role. Individual species differ across these large landscapes and species affinities are related to climatic suitability and soil characteristics. However, the ultimate expression of these exotic brome-grasses is related to ecosystem resistance to invasion, which is a function not only of environmental suitability but also of biotic interactions such as competition. Ongoing climate change is influencing ecosystem resistance and may ultimately result in a second generation of novel ecosystems dominated by different invaders. Due to the annual life form of many of these invaders, genetic adaptation may occur rapidly and can influence invasiveness and impacts. Consequences of invasion are dramatic influencing a suite of ecosystem processes ranging from altered disturbance regimes to changes in water storage, carbon sequestration, and nutrient cycling. Notably, invasion of annual brome-grasses increases amount and continuity of fine fuels in these arid and semi-arid ecosystems resulting in annual grass/fire cycles that decrease the capacity of longer-lived perennial natives to reestablish and favor annual bromes. Conversion to brome-grass dominance is placing numerous native species at risk, such as Greater Sage-Grouse, and threatening ecosystem services such as livestock forage, hunting and recreation, and even clean air and water. Viable approaches for restoring and maintaining native ecosystems have been developed, but the task becomes increasingly difficult with increases in either invasive grass dominance or aridity. Understanding human attitudes about novel ecosystems and the economic trade-offs is an essential part of restoring and maintaining native ecosystems.