The United States regulates deliberate species introduction by blacklists: any species not blacklisted may be imported. Half of invasive introduced species were deliberately introduced, yet most were not blacklisted, so this system is not working. White lists are also needed: no species can be deliberately introduced unless experts place it on a white list. The United States has not closed pathways for inadvertent introductions, which are regulated by international treaties. Risk assessments for introduced species have mostly targeted species as potential vectors for pathogens rather than as potentially invasive themselves. Although multilateral treaties mandate quantitative risk assessments for exclusions of species or goods that may carry them, biologists and economists can predict probabilities of invasiveness, and associated costs, only with enormous confidence limits. However, assessing risk can lead to insights on the likelihood of impact. The crucial decision will be to what extent the precautionary principle will be implemented. An added problem is that species established in parts of the United States (either native or introduced) have become invasive when introduced elsewhere in the United States. There is scant legal basis for preventing such introductions. To stem the flood of invasive species into and within the United States would require blacklists, a white listing procedure, and tighter regulation of pathways.