Current federal policy defines invasive species as those that are introduced into an ecological system by humans and then harm the environment, the economy, or human health. Current policy recognizes these species as a major concern and charges federal agencies to manage them to minimize the harm they cause. Over the past two decades, scientists have intensively studied the effects of invasive species on native species and on natural and agricultural systems and have estimated the environmental and economic costs and risks that invasive species impose. This symposium seeks to gather key results of this scientific research on the impacts of invasive species and to make it available to inform federal policy on this important environmental problem. Speakers from the disciplines of ecology and economics will address recent controversy over the impacts of invasive species and present syntheses of the valuation of harm, the point at which costs or risks should trigger action, and how harm compares in terrestrial and marine systems. A concluding panel discussion will work toward a useful definition of harm and a current statement about the harm that invasive species do.