Since at least the late 1800s, scientists and forest managers in the United States have recognized that forests have strong influences on water resources (Hewlett 2003). Today the connections between forests and water are widely reported in the news media, promoted by natural resource agencies, and investigated by specialists such as hydrologists, dendrologists, foresters, forest biologists, and water managers. Tree structures and functions influence at least four major areas of water-resources concerns: (1) water quality, (2) aquatic habitat, (3) water quantity, and (4) the interactions among water, climate, and energy use. This publication provides a basic introduction to Virginia's trees and a foundation for assessing the connections between water and trees.