Many natural resource systems such as lakes, rivers and streams, estuaries, and forests are used extensively for various kinds of recreation activities, including fishing, hunting, boating, hiking, and camping. As places to conduct such activities, natural resource systems provide valuable services. From an economic perspective, these services have two important features. First, the economic value of these services depends upon the characteristics of the natural resource system. The characteristics determining value can be affected by air and water pollution and by resource management decisions about such things as the rates of harvest of timber and fish, the extraction of minerals and petroleum, and the allocation of water flows between diversionary uses and various in-stream uses. Knowledge of the values of these services may be important for a variety of resource management decisions.