Technical Abstract: Medusahead invasion of Intermountain West rangelands reduces grazing capacity, degrades wildlife habitat, decreases biodiversity, and alters ecosystem functions. Medusahead invasion is at a critical threshold, either a large effort it is initiated to prevent its invasion of new areas or millions of hectares will be lost. Prevention of medusahead invasions should focus on three factors: 1) preventing medusahead seeds from dispersing to new locations, 2) increasing the resistance of desirable plant communities to medusahead invasion, and 3) searching for and eradicating new infestations. Initiating an endeavor to prevent and control new medusahead infestations will protect the economical and ecological yield of vast acreages. However, other invasive plants that have the potential to result in huge losses also need to be identified and comprehensive prevention programs will also have to be developed for these plants