Most ecosystem services that affect our quality of life are not bought and sold in the market, and thus we do not know what people would be willing to pay for them. What are the economic benefits of a decrease in air pollution? What are the economic costs of a Eurasian water-milfoil invasion? Environmental economists have honed several techniques to estimate such values, including Contingent Valuation (CV), hedonic price analysis, and the travel-cost method. NTL economists assessed the value of several ecosystem services in northern Wisconsin lakes currently threatened by human behavior, including green frog populations, milfoil prevention and control, fishing quality, and open space. Results suggest that lakeshore property-owners’ annual willingness-to-pay ranged from $52, to protect green frogs, to $1,380 to prevent milfoil from invading their lake. The hedonic and CV methods both converged on a total net present cost for a milfoil invasion of about $28,000 per household. This implies an aggregate per year loss of roughly $187,000 for one additional infested lake. For perspective, there are about 500 lakes in Wisconsin affected by milfoil and the State’s Department of Natural Resources allocates about $4.5 million annually for grants to control all aquatic invasive species.