Propagule pressure, or the number of nonnative individuals introduced into a system, has been assigned special importance in determining habitat invasibility. It has been suggested that propagule pressure may be the most important factor in establishment of nonnative species of various taxa in a variety of ecosystems worldwide. Strong evidence is emerging that propagule pressure strongly determines likelihood of invasion. This session springs from a 2006 workshop, convened by the US-EPA, on future research and polichy directions concerning aquatic nonnative invaders. Speakers will: discuss the outcomes of the summer 2006 workshop; present an overview of how propagule pressure affects the different stages of invasion; and discuss the association between propagule pressure and nonnative invasion at genetic, population, community, and ecosystem levels of biological organization. The session will suggest research directions addressing the most effective ways to stem the tide of invasive species. The current lack of tools for assessing the impact of nonnative aquatic invaders is a serious technological gap that strongly affects the capacity to screen for potential invasive nonnatives; speakers will suggest frameworks for the assessment of impact of nonnative aquatic invaders and for ranking threats. Speakers will present the links between propagule pressure and invasion impacts which may provide evidence to drive future national policies relating to the import and transport of nonnative species.