Managing the potential weed risks associated with the entry of new plant species into a country is dependent on a comprehensive regulatory system to control entry and a reliable system for predicting potential weediness in advance of release. The weed risk assessment (WRA) system developed in Australia has been used by many countries to assess weed risks. Evaluation of this system using test data sets of plants with known characteristics has shown that it performs well in identifying species that may be weedy but has a significant rate of false positives. However, the costs associated with allowing weedy species to establish and spread significantly exceed the benefits forgone by rejecting useful, non-weedy species particularly when longer time scales are considered.