The objective of this study is to undertake an economic analysis that will provide a better understanding of the economic value (to Ontario) of protecting the Great Lakes by preventing the establishment of AIS. The specific objective of this study is to gain an understanding of the magnitude of the impacts of an invasion relative to the costs of preventive measures. Two species groups are examined using a case study approach to assess costs: zebra mussels costs of invasion; and, Asian carp costs of prevention.