Aquatic ecosystem goods and services are the stocks and flows derived from ecosystem processes that have some value to humankind. These values may be direct and apparent, such as the economic value of fisheries, or they may be generated from indirect or unapparent means, such as the benefits accrued from ground water processes that purify drinking water. A number of methods are used to quantify aquatic and other ecosystem services, but this science is still in its infancy, and many issues are not resolved. Among the threats to ecosystem services are the familiar problems of habitat loss and fragmentation, pollution, and overuse, all of these exacerbated by the increasing populations of humans as well as their increasing patterns of consumption. Ecosystem service valuation is one way to make the ‘hidden subsidies of Nature’ visible and quantifiable.