The vast majority of the decisions made in our global society are responses to problems in which cause and effect are closely related in time and in space. When a problem becomes important, its source is usually obvious, and any appropriate response usually becomes effective in time to eliminate the difficulty. For this class of phenomena it is satisfactory to react after a problem is already apparent. Thus, the institutions involved need only monitor the current status of the system; they need not maintain a long planning horizon. It is important to realize that most environmental problems do not fit into this category. The delays associated with most environmental processes will require us to add an explicit consideration of the time dimension in formulating environmental policy.