For purposes of economic planning, management, and policymaking, it may often be useful to view environmental and natural resources as assets that yield flows of services over time. The theoretical framework necessary for defining and measuring the values of these service flows to the individuals who receive them has now been developed. However, this theory of value has been developed without explicit consideration of the temporal dimension of these service flows. Specifically, the theory has been applied to the values of services only at a single point in time or over some relatively short period of time (a day, week, or year) during which it is reasonable to treat other things such as income, prices, and the consumption of other goods and services as fixed. The theory also has defined values only at that moment in time at which the service flow is received; and thus, we are left with the question of how to aggregate values that are realized at different moments in time and that might vary from one moment to another.