Trees add a new growth layer, or ring, each year. Because growth is controlled in part by environment, analysis of tree rings can potentially yield information concerning past hydrologic (or other environmental) conditions. The present state of understanding of the relationship between tree growth and the hydrologic environment in the eastern United States does not yet allow detailed estimates of past environments from tree rings. It does appear though, that within the near future it will be possible to estimate certain hydrologic parameters in space (between gaging stations) from tree growth phenomena. For the present, however, examination of tree rings is most useful in hydrologic studies for dating of events.